Components for medical circuits

ABSTRACT

Breathable medical circuit components and materials and methods for forming these components incorporate breathable foamed materials that are permeable to water vapor and substantially impermeable to liquid water and the bulk flow of gases. The materials and methods can be incorporated into a variety of components, including tubes, Y-connectors, catheter mounts, and patient interfaces and are suitable for use in a variety of medical circuits, including insufflation, anesthesia, and breathing circuits.

PRIORITY

This utility application is a continuation of U.S. patent applicationSer. No. 13/517,925, filed Jun. 20, 2012, which is the U.S. nationalphase of International Application No. PCT/IB2010/003454, filed Dec. 22,2010, which claims the benefit of U.S. Provisional Patent ApplicationNo. 61/289,089, filed Dec. 22, 2009, the entire contents of each ofwhich are hereby incorporated by this reference.

BACKGROUND Field

This disclosure relates generally to components for medical circuits,and in particular to components for medical circuits providinghumidified gases to and/or removing humidified gases from a patient,such as in positive airway pressure (PAP), respirator, anaesthesia,ventilator, and insufflation systems.

Description of the Related Art

In medical applications, various components transport gases having highlevels of relative humidity to and from patients. Condensation, or “rainout,” can be a problem when the high humidity gases come into contactwith the walls of a component at a lower temperature. However,condensation is dependent on many factors, including not only thetemperature profile in the component, but also the gas flow rate,component geometry, and the intrinsic “breathability” of the materialused to form the component, that is the ability of the material totransmit water vapor, while substantially resisting the bulk flow ofliquid water and the bulk flow of gas.

For example, PAP systems (ventilation systems that provide patients withbreathing gases at positive pressure) use breathing tubes for deliveringand removing inspiratory and expiratory gases. In these applications,and in other breathing applications such as assisted breathing, thegases inhaled by a patient are usually delivered through an inspiratorytube at humidity near saturation. The breathing gases exhaled by apatient flow through an expiratory breathing tube and are usually fullysaturated. Condensation may form on the interior walls of a breathingcircuit component during patient inhalation, and significantcondensation levels may form during patient exhalation. Suchcondensation is particularly deleterious when it is in close proximityto the patient. For instance, mobile condensate forming in a breathingtube (either inspiratory or expiratory) can be breathed or inhaled by apatient and may lead to coughing fits or other discomfort.

As another example, insufflation systems also deliver and removehumidified gases. During laparoscopic surgery with insufflation, it maybe desirable for the insufflation gas (commonly CO₂) to be humidifiedbefore being passed into the abdominal cavity. This can help prevent“drying out” of the patient's internal organs, and can decrease theamount of time needed for recovery from surgery. Even when dryinsufflation gas is employed, the gas can become saturated as it picksup moisture from the patient's body cavity. The moisture in the gasestends to condense out onto the walls of the discharge limb or tube ofthe insufflation system. The water vapor can also condense on othercomponents of the insufflation system such as filters. Any vaporcondensing on the filter and run-off along the limbs (inlet or exhaust)from moisture is highly undesirable. For example, water that hascondensed on the walls can saturate the filter and cause it to becomeblocked. This potentially causes an increase in back pressure andhinders the ability of the system to clear smoke. Further, liquid waterin the limbs can run into other connected equipment, which isundesirable.

Attempts have been made to reduce the adverse effects of condensation byincorporating highly “breathable” materials—that is, materials that arehighly permeable to water vapor and substantially impermeable to liquidwater and the bulk flow of gases—into the tube walls. However, this hasrequired extremely thin membrane walls in order to achieve breathabilitysufficiently high to prevent or reduce condensation. As a result, tubeshaving acceptable breathability have had wall thicknesses so thin thatthe tubes need significant reinforcing measures. These reinforcingmeasures add time, cost, and complexity to the manufacturing process.Accordingly, a need remains for breathable, yet strong, components formedical circuits for delivering humidified gases.

SUMMARY

Materials and methods for forming breathable medical circuit components,such as breathable insufflation, anesthesia, or breathing circuitcomponents are disclosed herein in various embodiments. These breathablecomponents incorporate breathable foamed materials that are permeable towater vapor and substantially impermeable to liquid water and the bulkflow of gases. The disclosed materials and methods can be incorporatedinto a variety of components, including tubes, Y-connectors, cathetermounts, and patient interfaces.

A medical circuit component for use with humidified gas is disclosed. Inat least one embodiment, the component can comprise a wall defining aspace within and wherein at least a part of said wall is of a breathablefoamed material configured to allow the transmission of water vapor butsubstantially prevent the transmission of liquid water.

In various embodiments, the foregoing component has one, some, or all ofthe following properties. The diffusion coefficient of the breathablefoamed material can be at least 3×10⁻⁷ cm²/s. The thickness of the wallcan be between 0.1 mm and 3.0 mm The breathable foamed material cancomprise a blend of polymers. The breathable foamed material cancomprise a thermoplastic elastomer with a polyether soft segment. Thebreathable foamed material can comprise a copolyester thermoplasticelastomer with a polyether soft segment. The breathable foamed materialcan be sufficiently stiff, such that the foamed material can be bentaround a 25 mm diameter metal cylinder without kinking or collapse, asdefined in the test for increase in flow resistance with bendingaccording to ISO 5367:2000(E). The permeability P of the component ing-mm/m²/day can be at least 60 g-mm/m²/day when measured according toProcedure A of ASTM E96 (using the desiccant method at a temperature of23° C. and a relative humidity of 90%). The elastic modulus of thecomponent can be between 30 and 1000 MPa. The permeability P can satisfythe formula:

P>exp{0.019[1n(M)]²−0.710+1n(M)+6.5}

in which M represents the elastic modulus of the foamed polymer in MPaand M is between 30 and 1000 MPa.

In addition, in various embodiments, the component according to any orall of the preceding embodiments has one, some, or all of the followingproperties. The foamed material can comprise voids. The foamed materialcan have a void fraction greater than 25%. The foamed material can havean average void size in the transverse direction less than 30% of thewall thickness. The foamed material can comprise voids that areflattened along the wall's longitudinal axis. At least 80% of the voidscan have an aspect ratio of longitudinal length to transverse heightgreater than 2:1. At least 10% of the voids can be interconnected.

In certain embodiments, the component according to any or all of thepreceding embodiments can form the wall of a tube or the wall of a mask.If the foamed material forms the wall of a tube, the tube can be, forexample, an extruded tube, a corrugated tube, or an extruded, corrugatedtube. Any of these foregoing tubes can be a tube for use in aninsufflation system.

In at least one embodiment, the component can comprise a wall defining aspace, wherein at least a part of the wall is of a foamed material thatis permeable to water vapor and substantially impermeable to liquidwater, wherein the permeability P of the foamed material measuredaccording to Procedure A of ASTM E96 (using the desiccant method at atemperature of 23° C. and a relative humidity of 90%) in g-mm/m²/day isat least 60 g-mm/m²/day and satisfies the formula:

P>exp{0.019[1n(M)²−0.710+6.5}

-   wherein M represents the elastic modulus of the foamed material in    MPa and M is between 30 and 1000 MPa.

In various embodiments, the foregoing component has one, some, or all ofthe following properties. P can be at least 70 g-mm/m²/day. M can bebetween 30 and 800 MPa. The wall thickness can be between 0.1 mm and 3.0mm The foamed material can have a void fraction greater than 25%. Thefoamed material can comprise voids. The foamed material can have anaverage void size in the transverse direction less than 30% of the wallthickness. At least some of the voids can be flattened along the wall'slongitudinal axis. At least 80% of the voids can have an aspect ratio oflongitudinal length to transverse height greater than 2:1. At least 10%of the voids can be interconnected. The breathable foamed material cancomprise a thermoplastic elastomer with a polyether soft segment. Thebreathable foamed material can comprise a copolyester thermoplasticelastomer with a polyether soft segment.

In certain embodiments, the component according to any or all of thepreceding embodiments can form the wall of a tube or the wall of apatient's mask. If the foamed material forms the wall of a tube, thetube can be, for example, an extruded tube, a corrugated tube, or anextruded, corrugated tube. Any of these foregoing tubes can be a tubefor use in an insufflation system.

A method of manufacturing a medical circuit component is also disclosed.In at least one embodiment the method comprises mixing a foaming agentmasterbatch (a mixture of a carrier polymer and active foaming agent)into a polymeric base material and forming a liquefied mixture, allowingthe foaming agent portion to release gas bubbles into the base materialportion of the liquefied mixture, and arresting the release of gasbubbles and processing the mixture to form a water-vapor permeablecomponent.

In various embodiments, the foregoing method has one, some, or all ofthe following properties. The foaming agent and/or the polymeric basematerial can be selected and the mixture can be processed to form awater-vapor permeable component comprising a solid polymer and voidsdistributed throughout the solid polymer. The permeability P of thecomponent in g-mm/m²/day can be at least 60 g-mm/m²/day or least 70g-mm/m²/day measured according to Procedure A of ASTM E96 (using thedesiccant method at a temperature of 23° C. and a relative humidity of90%). The elastic modulus of the component can be between 30 and 1000MPa. P can satisfy the formula:

P>exp{0.019[1n(M)²−0.710+6.5}

-   in which M represents the elastic modulus of the foamed polymer in    MPa and M is between 30 and 1000 MPa, or between 30 and 800 MPa. The    wall thickness can be between 0.1 mm and 3.0 mm

In addition, in various embodiments, the method according to any or allof the preceding embodiments has one, some, or all of the followingproperties. The foamed material can comprise voids. The foamed materialcan have a void fraction greater than 25%. The average void size in thetransverse direction can be less than 30% of the wall thickness. Thefoamed material can comprise voids that are flattened along the wall'slongitudinal axis. At least 80% of the voids can have an aspect ratio oflongitudinal length to transverse height greater than 2:1. At least 10%of the voids can be interconnected. The breathable foamed material cancomprise a thermoplastic elastomer with a polyether soft segment. Thebreathable foamed material can comprise a copolyester thermoplasticelastomer with a polyether soft segment.

In certain embodiments, the method according to any or all of thepreceding embodiments can comprise forming the water-vapor permeablecomponent into a tube or forming the water vapor permeable componentinto a mask. If the method comprises forming the component into a tube,the act of processing the mixture can comprise extruding the mixtureinto a tube shape. Processing the mixture can also comprise co-extrudinga plurality of reinforcing ribs on a surface of the tube shape. The ribscan be arranged on an inner surface of the tube shape, or on an outersurface of the tube shape, or on the inner and outer surface of the tubeshape. In particular, the ribs can be arranged about the circumferenceof the tube shape, for example, circumferentially arranged about theinner surface of the tube shape. The ribs can be generallylongitudinally aligned along a length of the tube shape. Processing themixture can also comprise corrugating the extruded tube shape. If theextruded tube shape is corrugated, the tube shape can comprise ribs orthe ribs can be omitted.

A tube for delivering humidified gas to or from a patient is alsodisclosed. In at least one embodiment, the tube comprises an inlet andan outlet and an extruded, corrugated, foamed-polymer conduit that ispermeable to water vapor and substantially impermeable to liquid waterand bulk flow of gas, the foamed-polymer conduit being configured toenable flow of humidified gas from the inlet to the outlet within aspace enclosed by the conduit. The tube can further comprise a pluralityof reinforcing ribs. The ribs can be arranged on an inner surface of thetube shape, or on an outer surface of the tube shape, or on the innerand outer surface of the tube shape. In particular, the ribs can bearranged about the circumference of the tube shape, for example,circumferentially arranged about the inner surface of the tube shape.The ribs can be generally longitudinally aligned along a length of thetube shape between the inlet and the outlet.

In various embodiments, the foregoing tubes, either with or without theabove-described ribs, have one, some, or all of the followingproperties. The foamed-polymer conduit can comprise a solidthermoplastic elastomer material and cell voids distributed throughoutthe solid material. The foamed-polymer conduit can have an inner surfaceadjacent the enclosed space; and an inner volume adjacent the innersurface in which at least some of the cell voids are connected to othercell voids, thereby forming open cell pathways promoting movement ofwater vapor through the conduit. At least 10% or at least 20% of thecell voids can be connected to other cell voids. The inner volume canhave a void fraction greater than 25%. The average void size in thetransverse direction can be less than 30% of the wall thickness or lessthan 10% of the wall thickness. At least some of the voids can beflattened along a longitudinal axis of the conduit. The flattening canbe expressed as having an aspect ratio of longitudinal length totransverse height greater than 2:1 or greater than 3:1. At least 80% ofthe voids can have the flattening.

In addition, in various embodiments, the tube according to any or all ofthe preceding embodiments has one, some, or all of the followingproperties. The foamed-polymer conduit can have a wall thickness between0.1 mm and 3.0 mm The permeability P of the component in g-mm/m²/day canbe at least 60 g-mm/m²/day measured according to Procedure A of ASTM E96(using the desiccant method at a temperature of 23° C. and a relativehumidity of 90%). The elastic modulus of the component can be between 30and 1000 MPa. P can satisfy the formula:

P>exp{0.019[1n(M)²−0.710+6.5}

-   in which M represents the elastic modulus of the foamed polymer in    MPa and M is between 30 and 1000 MPa. The foamed polymer conduit can    be sufficiently stiff, such that the foamed polymer conduit can be    bent around a 25 mm diameter metal cylinder without kinking or    collapse, as defined in the test for increase in flow resistance    with bending according to ISO 5367:2000(E).

In at least one embodiment, the tube comprises an inlet and an outletand a foamed-polymer conduit that is permeable to water vapor andsubstantially impermeable to liquid water and bulk flow of gas, suchthat the foamed-polymer conduit enables flow of humidified gas from theinlet to the outlet within a space enclosed by the conduit, wherein thefoamed-polymer conduit comprises a solid thermoplastic elastomermaterial and cell voids distributed throughout the solid material. Thefoamed-polymer conduit can have an inner surface adjacent the enclosedspace; and an inner volume adjacent the inner surface. At least some ofthe cell voids in the inner volume can be connected to other cell voids,thereby forming open cell pathways promoting movement of water vaporthrough the conduit.

In various embodiments, the foregoing tubes have one, some, or all ofthe following properties. The foamed-polymer conduit can have adiffusion coefficient greater than 3×10⁻⁷ cm²/s. The conduit can beextruded. The conduit can be corrugated. The tube can further comprise aplurality of reinforcing ribs. The ribs can be arranged on an innersurface of the tube shape, or on an outer surface of the tube shape, oron the inner and outer surface of the tube shape. In particular, theribs can be arranged about the circumference of the tube shape, forexample, circumferentially arranged about the inner surface of the tubeshape. The ribs can be generally longitudinally aligned along a lengthof the tube shape between the inlet and the outlet. The tube cancomprise a heating line. The line can be generally longitudinallyaligned along a length of the foamed-polymer conduit between the inletand the outlet.

In addition, in various embodiments, the tube according to any or all ofthe preceding embodiments has one, some, or all of the followingproperties. At least 10% or at least 20% of the cell voids in the innervolume can be connected to other cell voids. The inner volume can have avoid fraction greater than 25%. At least some of the voids can beflattened along a longitudinal axis of the conduit. The flattening canbe expressed as having an aspect ratio of longitudinal length totransverse height greater than 2:1, or greater than 3:1. At least 80% ofthe voids can have the flattening. The inner volume can have an averagevoid size in the transverse direction less than 30% or less than 10% ofthe foamed-polymer conduit wall thickness. The fomed-polymer conduit canhave a wall thickness between 0.1 mm and 3.0 mm The permeability P ofthe tube in g-mm/m²/day can be at least 60 g-mm/m²/day measuredaccording to Procedure A of ASTM E96 (using the desiccant method at atemperature of 23° C. and a relative humidity of 90%). The elasticmodulus of the tube can be between 30 and 1000 MPa P can satisfy theformula:

P>exp{0.019[1n(M)²−0.710+6.5}

-   in which M represents the elastic modulus of the foamed polymer in    MPa. The foamed polymer conduit can further have an outer skin    adjacent the inner volume in which the cell voids are closed cell.    The skin thickness can be between 5 and 10% of the wall thickness,    for example, between 10 and 50 μm.

A method of manufacturing a tube suitable for delivering humidified gasto or from a patient is also disclosed. In at least one embodiment, themethod comprises mixing a foaming agent into a base material to form anextrudate, the base material comprising one or more thermoplasticelastomers; applying pressure to the extrudate using an extruder to forma hollow tube; delivering the hollow tube to a corrugator mold; allowingthe hollow tube to cool within the corrugator mold; and removing thecooled hollow tube from the corrugator, thereby forming a corrugatedwater-vapor permeable tube.

In various embodiments, the foregoing method has one, some, or all ofthe following properties. The tube can have a wall thickness between 0.1mm and 3.0 mm The corrugated tube can comprise solid thermoplasticelastomer and voids formed by the gas bubbles released by the foamingagent. The maximum void size diameter in the transverse direction can beless than one third of the minimum wall thickness. The void fraction ofthe corrugated tube can be greater than 25%. The base material can havea diffusion coefficient greater than 0.75×10⁻⁷ cm²/s. The base materialcan have a tensile modulus greater than 15 MPa.

A method of delivering humidified gas to or from a patient is alsodisclosed. In at least one embodiment, the method comprises providing amedical circuit component comprising a wall formed of breathable foamedmaterial, connecting the medical circuit component to a patient, andtransmitting humidified gas via the medical circuit component, whereinthe medical circuit component allows for the passage of water vaporthrough the wall of the component but substantially prevents thetransmission of liquid water and bulk flow of gas through the wall ofthe component.

In various embodiments, the foregoing method has one, some, or all ofthe following properties. The diffusion coefficient of the breathablefoamed material can be at least 3×10⁻⁷ cm²/s. The thickness of the wallcan be between 0.1 mm and 3.0 mm The breathable foamed material cancomprise a thermoplastic elastomer with a polyether soft segment. Inparticular, the breathable foamed material can comprise a copolyesterthermoplastic elastomer with a polyether soft segment. The breathablefoamed material can be sufficiently stiff, such that the foamed materialcan be bent around a 25 mm diameter metal cylinder without kinking orcollapse, as defined in the test for increase in flow resistance withbending according to ISO 5367:2000(E). The permeability P of thecomponent in g-mm/m²/day can be at least 60 g-mm/m²/day when measuredaccording to Procedure A of ASTM E96 (using the desiccant method at atemperature of 23° C. and a relative humidity of 90%). The elasticmodulus of the component can be between 30 and 1000 MPa. P can satisfythe formula:

P>exp{0.019[1n(M)²−0.710+6.5}

-   in which M represents the elastic modulus of the foamed polymer in    MPa and M is between 30 and 1000 MPa.

In addition, in various embodiments, the method according to any or allof the preceding embodiments has one, some, or all of the followingproperties. The foamed material can comprise voids. At least 10% of thevoids can be interconnected. The foamed material can have a voidfraction greater than 25%. The foamed material can have an average voidsize in the transverse direction less than 30% of the wall thickness. Atleast some of the voids can be flattened along a longitudinal axis ofthe component. The flattening can be expressed as having an aspect ratioof longitudinal length to transverse height greater than 2:1 or greaterthan 3:1. At least 80% of the voids can have the flattening.

In certain embodiments, transmitting humidified gas via the medicalcircuit component can comprise transmitting humidified gas through atube comprising the breathable foamed material, or transmittinghumidified gas via a mask comprising the breathable foamed material, ortransmitting humidified gas via an insufflation tube comprising thebreathable foamed material.

The invention comprises all of the foregoing embodiments and alsocontemplates constructions of the following examples.

BRIEF DESCRIPTION OF THE DRAWINGS

Example embodiments that implement the various features of the disclosedsystems and methods will now be described with reference to thedrawings. The drawings and the associated descriptions are provided toillustrate embodiments and not to limit the scope of the disclosure.

FIG. 1 is a schematic illustration of a medical circuit incorporatingbreathable components.

FIG. 2A is a log/log plot of permeability versus Young's modulus forseveral previously known breathable materials used for components inmedical circuits; and FIG. 2B is a log/ log plot of permeability vs.Young's modulus for previously known materials, and for breathablefoamed polymer materials according to embodiments discussed herein.

FIG. 3 is a plot of relative diffusivity versus void fraction inbreathable foamed polymer materials according to embodiments discussedherein.

FIGS. 4A through 4D are micrographs of an example foamed, corrugatedtube; FIGS. 4E and 4F are micrographs of another example foamed,corrugated tube; FIGS. 4G and 4H are micrographs of an example foamed,extruded strip; FIGS. 4I and 4J are micrographs of another examplefoamed, extruded strip; FIG. 4K is a micrograph of a non-foamed,extruded strip formed from a polymer blend; FIGS. 4L and 4M aremicrographs of a foamed, extruded strip formed from the polymer blend;and FIGS. 4N and 4O are micrographs a non-foamed, extruded polymerstrip.

FIG. 5 is a schematic illustration of a component for a medical circuitincorporating a breathable foamed polymer material.

FIG. 6A is a side-plan view of a tubular component incorporating abreathable foamed polymer material; and FIG. 6B is cross section view ofthe tube component of FIG. 6A.

FIG. 7A is a front-perspective view of a tubular component incorporatingintegral, reinforcing ribs, the component being partially corrugated;FIG. 7B is a front perspective view of the tubular component being fullycorrugated.

FIG. 8A is a front-perspective photograph of an alternate configurationof a corrugated, tubular component incorporating ribs; FIG. 8B is afront-perspective photograph of the tubular component of FIG. 8A; andFIG. 8C is a corrugator block suitable for forming the tubular componentof FIGS. 8A and 8B.

FIG. 9 is a schematic illustration of a breathing circuit according toat least one embodiment.

FIG. 10 is a schematic illustration of a component comprising a coaxialtube, according to at least one embodiment.

FIG. 11A is a side-plan view of a mask-type patient interface accordingto at least one embodiment; and FIG. 11B is a front perspective view ofthe patient interface of FIG. 11A.

FIG. 12 is a front perspective view of a patient wearing anasal-cannula-type patient interface according to at least oneembodiment.

FIG. 13 is a schematic illustration of a catheter mount according to atleast one embodiment.

FIG. 14 is a schematic illustration of a humidified insufflation systemaccording to at least one embodiment, comprising inlet and exhaustlimbs.

FIG. 15 is a schematic illustration of a method of manufacturing acomponent according to at least one embodiment.

FIGS. 16A and 16B are micrographs showing an extruded foam polymerhaving an outer skin layer.

FIG. 17 is a flow chart showing a method of manufacturing a componentaccording to at least one embodiment.

FIG. 18 is a plot of an ideal sorption/desorption curve with constantdiffusivity.

FIG. 19 is a plot of representative experimental desorption curves.

FIG. 20 is a plot of experimental versus calculated desorption curve.

Throughout the drawings, reference numbers are re-used to indicatecorrespondence between referenced (or similar) elements. In addition,the first digit of each reference number indicates the figure in whichthe element first appears.

DETAILED DESCRIPTION

The following detailed description discloses new materials and methodsfor forming breathable medical circuit components, such as breathableinsufflation, anesthesia, or breathing circuit components. As explainedabove, these breathable components are permeable to water vapor andsubstantially impermeable to liquid water and the bulk flow of gases.The disclosed materials and methods can be incorporated into a varietyof components, including tubes (e.g., inspiratory breathing tubes andexpiratory breathing tubes and other tubing between various elements ofa breathing circuit, such as ventilators, humidifiers, filters, watertraps, sample lines, connectors, gas analyzers, and the like),Y-connectors, catheter mounts, and patient interfaces (e.g., masks forcovering the nose and face, nasal masks, cannulas, nasal pillows, etc.),in a variety of medical circuits. Medical circuit is a broad term and isto be given its ordinary and customary meaning to a person of ordinaryskill in the art (that is, it is not to be limited to a special orcustomized meaning). Thus, a medical circuit is meant to include opencircuits, such as certain CPAP systems, which can comprise a singleinspiratory breathing tube between a ventilator/blower and a patientinterface, as well as closed circuits.

Breathing Circuit Comprising Breathable Components

For a more detailed understanding of the disclosure, reference is firstmade to FIG. 1, which shows a breathing circuit according to at leastone embodiment, which includes one or more breathable components. Such abreathing system can be a continuous, variable, or bi-level positiveairway pressure (PAP) system or other form of respiratory therapy. Inthe example breathing circuit, a patient 101 receives humidified gas viaa breathable inspiratory tube 103. Tube is a broad term and is to begiven its ordinary and customary meaning to a person of ordinary skillin the art (that is, it is not to be limited to a special or customizedmeaning) and includes, without limitation, non-cylindrical passageways.An inspiratory tube is a tube that is configured to deliver humidifiedbreathing gases to a patient. Breathable tubes are discussed in greaterdetail below.

Humidified gases can be transported in the circuit of FIG. 1 as follows.Dry gases pass from a ventilator/blower 105 to a humidifier 107, whichhumidifies the dry gases. The humidifier 107 connects to the inlet 109(the end for receiving humidified gases) of the inspiratory tube 103 viaa port 111, thereby supplying humidified gases to the inspiratory tube103. The gases flow through the inspiratory tube 103 to the outlet 113(the end for expelling humidified gases), and then to the patient 101through a patient interface 115 connected to the outlet 113. Anexpiratory tube 117 also connects to the patient interface 115. Anexpiratory tube is a tube that is configured to move exhaled humidifiedgases away from a patient. Here, the expiratory tube 117 returns exhaledhumidified gases from the patient interface 115 to the ventilator/blower105.

In this example, dry gases enter the ventilator/blower 105 through avent 119. A fan 121 can improve gas flow into the ventilator/blower bydrawing air or other gases through vent 119. The fan 121 can be, forinstance, a variable speed fan, where an electronic controller 123controls the fan speed. In particular, the function of the electroniccontroller 123 can be controlled by an electronic master controller 125in response to inputs from the master controller 125 and a user-setpredetermined required value (preset value) of pressure or fan speed viaa dial 127.

The humidifier 107 comprises a humidification chamber 129 containing avolume of water 130 or other suitable humidifying liquid. Preferably,the humidification chamber 129 is removable from the humidifier 107after use. Removability allows the humidification chamber 129 to be morereadily sterilized or disposed. However, the humidification chamber 129portion of the humidifier 107 can be a unitary construction. The body ofthe humidification chamber 129 can be formed from a non-conductive glassor plastics material. But the humidification chamber 129 can alsoinclude conductive components. For instance, the humidification chamber129 can include a highly heat-conductive base (for example, an aluminumbase) contacting or associated with a heater plate 131 on the humidifier107.

The humidifier 107 can also include electronic controls. In thisexample, the humidifier 107 includes an electronic, analog or digitalmaster controller 125. Preferably, the master controller 125 is amicroprocessor-based controller executing computer software commandsstored in associated memory. In response to the user-set humidity ortemperature value input via dial 133, for example, and other inputs, themaster controller 125 determines when (or to what level) to energizeheater plate 131 to heat the water 130 within humidification chamber129.

Any suitable patient interface 115 can be incorporated. Patientinterface is a broad term and is to be given its ordinary and customarymeaning to a person of ordinary skill in the art (that is, it is not tobe limited to a special or customized meaning) and includes, withoutlimitation, masks (such as face masks and nasal masks), cannulas, andnasal pillows. A patient interface usually defines a gases space which,when in use, receives warm humid breathing gases and is therefore atrisk of rain out forming. Due to the close proximity of the patientinterface 115 to the patient 101, this is very undesirable. To addressthe risk of rain out, a temperature probe 135 can connect to theinspiratory tube 103 near the patient interface 115, or to the patientinterface 115. The temperature probe 135 monitors the temperature nearor at the patient interface 115. A heating line (not shown)communicating with the temperature probe can be used to adjust thetemperature in the patient interface 115 and/or inspiratory tube 103 toraise the temperature in the inspiratory tube 103 and/or patientinterface 115 above the saturation temperature. In addition to (or as analternative to) a temperature probe and heating line, the patientinterface 115 can also comprise a breathable interface, as described ingreater detail below with respect to FIGS. 11A, 11B, and 12.

In FIG. 1, exhaled humidified gases are returned from the patientinterface 115 to the ventilator/blower 105 via the expiratory tube 117.The expiratory tube 117 preferably comprises a breathable foamedmaterial, as described below. However, the expiratory tube 117 can alsobe a medical tube as previously known in the art. In either case, theexpiratory tube 117 can have a temperature probe and/or heating line, asdescribed above with respect to the inspiratory tube 103, integratedwith it to reduce the risk of rain out. Furthermore, the expiratory tube117 need not return exhaled gases to the ventilator/blower 105.Alternatively, exhaled humidified gases can be passed directly toambient surroundings or to other ancillary equipment, such as an airscrubber/filter (not shown). In certain embodiments, the expiratory tubeis omitted altogether.

Foamed Polymers for Forming Breathable Components

As explained above with respect to FIG. 1, medical circuits such asbreathing circuits can make use of breathable components, such as tubesor patient interfaces. Breathability is desirable to prevent rain out inthese components. One measure of the material breathability ispermeability (expressed in g-mm/m²/day). Another measure ofbreathability is the diffusivity of water in the material (diffusioncoefficient, measured in cm²/sec). At similar test conditions, forexample at similar temperatures, a given material's permeability anddiffusivity are directly proportional to each other. It is known thatbreathable thermoplastic elastomer materials (TPE according to ISO18064:2003(E), which is hereby incorporated in its entirety by thisreference) are particularly suited to forming these breathablecomponents. However, these known materials are flimsy and requiresubstantial reinforcement to render them usable.

It was found that the breathability-to-strength relationship can beunexpectedly improved by foaming polymer materials, including previouslyknown breathable polymers, as they are formed into components. Byincorporating highly breathable foamed material, components can bemanufactured having both a relatively high flexural stiffness and a highbreathability. Similarly, components formed from the foamed materialdescribed herein can also have relatively high resistance to crushingand resistance to buckling. As a result, it is possible to manufacturetubes with adequate “bulk” properties (for example, thickness, material,material blending, elastic modulus, breathability, and/or bulkstiffness) to meet the requirements of the ISO 5367:2000(E) standard(namely, the test for increase in flow resistance) without extrareinforcing, and also to be sufficiently breathable as defined in moredetail later. ISO 5367:2000(E) is hereby incorporated in its entirety bythis reference. For instance, it has been found that breathablethermoplastic elastomer (TPE) materials, such as ARNITEL® VT 3108, areparticularly suited to foaming and forming components according variousembodiments. For this material, the breathability-to-strengthrelationship can be significantly improved by foaming the material as itis formed into a product or component.

Thus, certain embodiments include the realization that particular foamedpolymers can be used to form breathable components, such that thecomponents have combined Young's modulus (stiffness) and permeability(breathability) properties that are significantly improved overpreviously known breathable materials. These new foamed polymers andtechniques for forming the foamed polymers and medical circuitcomponents incorporating such foamed polymers are described herein asillustrative examples. Because of their high permeability, these foamedpolymers allow water vapor to diffuse through them rapidly. This reducesthe build up of condensation within the component by transmitting watervapor from the humidified gases within the component to the surroundingambient air or to other drier gases on the other side of the component.Yet, the components formed from these foamed polymers are also stiff,self supporting, crush resistant, or semi-rigid, and even may notrequire additional reinforcement. The foamed polymers are useful forforming medical circuit components because the foamed polymer allow thetransmission of water vapor from gases, but prevent the transmission ofliquid water. They are also substantially impermeable to the bulk flowof gas, such that they can be used to form components for deliveringhumidified gases.

In general, the foamed polymer according to at least one embodiment is abreathable foamed thermoplastic polymer. Preferably, the breathablethermoplastic polymer is a foamed thermoplastic elastomer (or TPE asdefined by ISO 18064:2003(E)), such as (1) a copolyester thermoplasticelastomer (e.g., ARNITEL®, which is a copolyester thermoplasticelastomer with a polyether soft segment, or other TPC or TPC-ETmaterials as defined by ISO 18064:2003(E)), or (2) a polyether blockamide (e.g., PEBAX®, which is a polyamide thermoplastic elastomer with apolyether soft segment, or other TPA-ET materials as defined by ISO18064:2003(E)), or (3) a thermoplastic polyurethane (TPU material asdefined by ISO 18064:2003(E)), or (4) a foamed polymer blend, such as aTPE/polybutylene terephthalate (PBT, e.g., DURANEX® 500FP) blend. If thebreathable thermoplastic polymer is a foamed TPE/PBT blend, the blendpreferably comprises between 80% and 99% (or about 80% and 99%) TPE byweight and 20% and 1% (or about 20% and 1%) PBT by weight.

In any of the above embodiments, the void fraction of the foamedmaterial can be greater than 25% (or about 25%), such as between 25 and60% (or about 25 and 60%), or between 30 and 50% (or about 30 and 50%).In at least one embodiment, no more than 5% (or about 5%) of the voidsof said foamed material exceed a diameter of 500 μm.

FIG. 2A shows a log/log plot of literature values of permeability versusYoung's modulus for breathable materials previously known in the art.The values vary over six orders of magnitude in both modulus andpermeability.

FIG. 2B adds to FIG. 2A data points for examples of example foamedpolymers according to various embodiments disclosed herein, labelled #1through #4 and #6. It was discovered that the combined permeability andmodulus for all the previously known materials did not exceed line 201,representing the formula:

1n(P)=0.019(1n(M))²−0.7 1n(M)+6.5

-   in which P represents permeability of the material in g·mm/m²/day,    measured according to ASTM E96 Procedure A (desiccant method at a    temperature of 23° C. and a relative humidity of 90%), and M    represents the Young's modulus of the material in MPa. ASTM E96 is    hereby incorporated in its entirety by this reference.

For the foamed polymer materials represented by points #1 through #4,#6, and #8 in FIG. 2B, permeability P satisfies the formula:

P>exp{0.019[1n(M)]²−0.7 1n(M)+6.5}

-   Thus, these foamed polymers have combined levels of breathability    and stiffness not previously known.

The permeability and modulus of a foamed polymer can be selected toprovide improved stiffness and/or breathability in componentsincorporating the foamed polymer. Preferably, the material should bestiff enough to not easily crush or kink or change volume with pressure.For example, the breathable foamed polymer should be sufficiently stiff,such that the foamed polymer can be bent around a 25 mm diameter metalcylinder without kinking or collapse, as defined in the test forincrease in flow resistance with bending according to ISO 5367:2000(E).Therefore, modulus M is greater than 30 MPa (or about 30 MPa) in atleast one embodiment. The line M=30 MPa is indicated on FIG. 2B as line203. However, it may also be desirable to limit the stiffness of thecomponent, to make the component easier to handle or improve patientcomfort. Therefore, modulus M can be limited in certain embodiments toless than 1000 MPa (or about 1000 MPa). The line M=1000 MPa is indicatedas line 205. It may also be desirable to limit the modulus M to lessthan 800 MPa (or about 800 MPa), or less than 500 MPa (or about 500MPa).

In addition, it may be desirable to select a breathability sufficientlyhigh to prevent condensation in a variety of common uses and medicalcomponents. It was discovered that the diffusivity of a foamed polymeris function of void volume fractions. This is illustrated in TABLE 1,which summarizes at each relative humidity (RH) the ratio of thediffusivity at a specific void fraction (D) divided by the diffusivity,at the same RH, of solid ARNITEL® VT 3108 (D₀). The plot of the data inTABLE 1 is shown in FIG. 3.

TABLE 1 VALUES OF RELATIVE DIFFUSIVITY D/D₀ Point # Sample Name VoidFraction RH = 100 RH = 97 RH = 92 RH = 84 RH = 75 RH = 69 FmdAd1 0.1681.18 1.09 1.19 1.17 1 AB-14.2 0.337 2.25 2.26 2.27 2 MB 27 4% 0.41 2.612.68 2.73 2.64 3.21 3 FIIA-2 0.466 4.28 4.11 3.45 2.95 4 FIIA-5 0.536.60 7.13 7.79 4 FIIA-5 0.53 7.08 5.90 7 FIIA-1 0 1.00 1.00 1.00 1.001.00 1.00 Batch 15 wts 0.56 7.19 Batch 15 wts 0.56 7.44 6.83 6.36 6.98Batch 15 f 0.55 4.97 5.37 6.02 5.99 6.06 MB 27 6% 0.52 5.60 5.19 5.526.17 6.95 MB 41.4 0.462 3.91 3.59 3.74 4.06 MB 22.1 0.241 1.65 1.66 1.56

Accordingly, it is possible to select an appropriate level ofpermeability and/or void fraction for the foamed polymer to define anappropriate breathability. In certain embodiments, permeability P isgreater than 60 g-mm/m²/day (or about 60 g-mm/m²/day), measuredaccording to ASTM E96 Procedure A. A permeability of 60 g-mm/m²/dayrepresents a 66% increase over solid ARNITEL® VT 3108. The line P=60g-mm/m²/day is indicated as line 207. It may also be desirable to selecta permeability P greater than 70 MPa g-mm/m²/day (or about 70g-mm/m²/day) in some embodiments.

It is possible to relate permeability to a corresponding void fraction.A permeability of 60 g-mm/m²/day is 1.66 times the value of solidARNITEL® VT 3108. Knowing that permeability is directly proportional todiffusivity, then it is possible to seek a corresponding void fractionwhere the diffusivity ratio is greater than 1.66 from FIG. 3. From FIG.3, the corresponding void fraction is greater than 25%. Accordingly, incertain embodiments, void fraction is greater than 25% (or about 25%).It may also be desirable to select a void fraction greater than 30% (orabout 30%) in some embodiments. A void fraction of 30% corresponds witha permeability of 70 g-mm/m²/day (or about 70 g-mm/m²/day), as explainedabove.

It can also be desirable to limit the void fraction in the foamedpolymer, to prevent liquid water from leaking through the voids. If thefoamed polymer does not have an outer skin structure (discussed ingreater detail below), then it may be desirable to have a void fractionless than 45% (or about 45%). If the foamed polymer has an outer skinstructure, then a void fraction less than 60% (or about 60%) may besuitable. It has been found that a void fraction between 25 and 60% (orabout 25 and 60%) for foamed ARNITEL® VT 3108 is suitable for formingcomponents for medical circuits as described herein. For example, a voidfraction of 30% (or thereabout) can improve the breathability of ArnitelVT 3108 by up to 2 times. A relatively modest modulus decrease can beoffset by added thickness of the component as described below, whilestill maintaining a similar breathability. It has been found that a voidfraction between 30 and 50% (or about 30 and 50%) of foamed ARNITEL® VT3108 is particularly well suited for forming these components. It willbe appreciated that the foregoing are only examples of suitable voidfraction percentages and the corresponding material properties.

As discussed above, another measure of the material breathability is thediffusivity of water in the material (diffusion coefficient, measured incm²/sec). At similar test conditions, permeability and diffusivity aredirectly proportional to each other for a specific base material. Invarious embodiments, the foamed polymer has a diffusion coefficientgreater than 3×10⁻⁷ cm²/s (or thereabout), and more preferably greaterthan 6×10⁻⁷ cm²/s (or thereabout). For example, a 0.1625 cm diameter rodof foamed ARNITEL® VT 3108 at 47% void fraction has been calculated tohave a diffusion coefficient equal to (or about equal to) 7.6×10⁻⁷cm²/s. As another example, a 0.0505 cm thick film of foamed ARNITEL® VT3108 at 13% void fraction has been calculated to have a diffusioncoefficient equal to (or about equal to) 3.3×10⁻⁷ cm²/sec.

Samples #1 through #4 in FIG. 2B comprise foamed ARNITEL® VT 3108. Itcan be seen that these materials, and particularly sample #4 at 53% voidfraction, perform better than any other previously known material interms of their combined permeability and modulus. For sample #4, thefoaming process resulted in nearly a 6.5-fold average increase inpermeability at 97% RH, while still having a modulus 30% of pureARNITEL® VT 3108.

In FIG. 2B, point #1 represents data for a sample named “AB 14.2a.” AB14.2a is a foamed, corrugated ARNITEL® VT 3108 adult tube with an outerdiameter of 24.5 cm. Experimental data collected on this sample includephotomicrographs (shown in FIGS. 4A through 4D and summarized in TABLE2), void fraction and average sample thickness (shown in TABLE 3),modulus (shown in TABLE 4), and the variation of diffusivity with RH(summarized in TABLE 1).

Point #2 represents data for a sample named “MB27 4%.” MB27 4% is afoamed, corrugated ARNITEL® VT 3108 infant tube with an outer diameterof 15.46 cm. The tube was extruded from a mixture of a base polymer(ARNITEL® VT 3108) and 4% (or about 4%) by weight of a foaming agentmasterbatch (comprising polyethylene and 20% by weight of ClariantHYDROCEROL® BIH-10E). Experimental data collected on this sample includephotomicrographs (shown in FIGS. 4E and 4F and summarized in TABLE 2),void fraction and average sample thickness (shown in TABLE 3), modulus(shown in TABLE 4), and the variation of diffusivity with RH (summarizedin TABLE 1).

Point #3 represents data for sample named “FIIA-2.” FIIA-2 is a foamedextruded strip of ARNITEL® VT 3108. Experimental data collected on thissample include photomicrographs (shown in FIGS. 4G and 4H and summarizedin TABLE 2), void fraction and average sample thickness (shown in TABLE3), modulus (shown in TABLE 4), and the variation of diffusivity with RH(summarized in TABLE 1). The variation of dimensions with water contentwas also measured. It was determined that the variation of length withwater content can be described by the following equation:

${\Delta \frac{X}{X_{0}}} = {{0.3683( {W\mspace{14mu} \%} )} - {0.1626( {W\mspace{14mu} \%} )^{2}}}$

where

W % is the grams of water absorbed per gram dry polymer

X is the measured dimension, and

X₀ is the measured dimension at W %=0.

Point #4 represents data for a sample named “FIIA-5.” FIIA-5 is a foamedextruded strip of ARNITEL® VT 3108. Experimental data collected on thissample include photomicrographs (shown in FIGS. 4I and 4J and summarizedin TABLE 2), void fraction and average sample thickness (shown in TABLE3), modulus (shown in TABLE 4), and the variation of diffusivity with RH(summarized in TABLE 1). The variation of dimensions with water contentwas also measured. It was determined that the variation of length withwater content (Δ X/X₀) can be described by the following equation:

${\Delta \frac{X}{X_{0}}} = {{0.3674( {W\mspace{14mu} \%} )} - {0.3012( {W\mspace{14mu} \%} )^{2}}}$

Point #5 represents data for a sample named “80/20 ARNITEL/PBT.” 80/20ARNITEL/PBT is an extruded strip of polymer made from a 80/20 weightpercent blend of ARNITEL® VT 3108 and polybutylene terephthalate (PBT).Experimental data collected on this sample include photomicrographs(shown in FIG. 4K and summarized in TABLE 2), average sample thickness(shown in TABLE 3), modulus (shown in TABLE 4), and the diffusivityRH=100 (summarized in TABLE 1).

Point #6 represents data for a sample named “80/20 ARNITEL/PBT foamed.”80/20 ARNITEL/PBT foamed is a foamed extruded strip of polymer made froma 80/20 weight percent blend of ARNITEL® VT 3108 and PBT. Experimentaldata collected on this sample include photomicrographs (shown in FIGS.4L and 4M and summarized in TABLE 2), void fraction and average samplethickness (shown in TABLE 3), modulus (shown in TABLE 4), and thediffusivity at RH=100 (summarized in TABLE 1).

Point #7 represents data for a sample named “FIIA-1.” FIIA-1 is anextruded strip of solid Arnitel 3108. Experimental data collected onthis sample include photomicrographs (shown in FIGS. 4N and 4O andsummarized in TABLE 2), average sample thickness (shown in TABLE 3),modulus (shown in TABLE 4), and the variation of diffusivity with RH(summarized in TABLE 1). The variation of dimensions with water contentwas also measured. Variation of all three dimensions (length, width andthickness) with water content were observed to be nearly identical(i.e., isotropic expansion) and could be described the followingequation.

${\Delta \frac{X}{X_{0}}} = {{0.4123( {W\mspace{14mu} \%} )} - {0.1410( {W\mspace{14mu} \%} )^{2}}}$

This relationship was used to calculate the variation of samplethickness in time in water desorption experiments.

Finally, point #8 represents data for a sample named “TPU/Acetal fmd10%.” TPU-acetal fmd 10% is an extruded strip of a foamed blend ofESTANE® 58245 (a TPU) and acetal. Experimental data collected on thissample included void fraction and average sample thickness (shown inTABLE 3), modulus (shown in TABLE 4), and diffusivity (shown in TABLE4).

Also shown in FIG. 2B is a point labelled “FmdAd1.” FmdAd1 is a foamed,corrugated ARNITEL® VT 3108 adult tube with an outside diameter of 24.5cm. Experimental data collected on this sample included void fractionand average sample thickness (shown in TABLE 3), modulus (shown in TABLE4), and the variation of diffusivity with RH (summarized in TABLE 1).

Additional unfoamed and foamed polymer materials that are not plotted inFIG. 2A or 2B are described below.

“Batch 15 wts,” “Batch 15 f,” “MB27 0%,” “MB27 6%,” “MB22.1,” “MB32.1,”and “MB41.4” are foamed, corrugated ARNITEL® VT 3108 infant tubes withan outer diameter of 15.46 cm. Experimental data collected on thesesample included void fraction and average sample thickness (shown inTABLE 3) and the variation of diffusivity with RH (summarized in TABLE1). For MB32.1, the variation of length with water content was alsomeasured. The variation was found to be described by the equation:

${\Delta \frac{X}{X_{0}}} = {{0.4614( {W\mspace{14mu} \%} )} - {0.1742( {W\mspace{14mu} \%} )^{2}}}$

“TPU, ESTANE 58245” is an unfoamed, corrugated, TPU (ESTANE® 58245) tubehaving a wall thickness of 0.048 cm. Experimental data collected on thissame included void fraction and average sample thickness (shown in TABLE3), modulus (shown in TABLE 4), and diffusivity (shown in TABLE 4).

TABLE 2 SUMMARY OF MICROGRAPHS Point # Sample Name MagnificationsComment 1 AB 14.2a 10x (FIGS. 4A, C) Many cells flattened, 20x (FIGS.4B, D) several connected 2 MB27 4% 10x (FIG. 4E) All cells veryflattened, 20x (FIG. 4F) many connected 3 FIIA-2 20x (FIG. 4G) Veryflattened cells, 30x (FIG. 4H) many connected 4 FIIA-5 20x (FIG. 4I)Very flattened cells, 30x (FIG. 4J) many connected 5 80/20 20x (FIG. 4K)No cells observed ARNITEL/PBT 6 80/20 15x (FIG. 4L) Cells are sphericaland ARNITEL/PBT 20x (FIG. 4M) isolated from each other foamed 7 FIIA-120x (FIG. 4N) No cells observed 30x (FIG. 4O)

The micrographs show that the foamed polymer samples (samples #1 through#4 and #6) comprise cells or voids within solid polymer. Desirably thesize of these voids in the transverse direction are less than 30% (orabout 30%) of the thickness of the foamed polymer, for example, lessthan 10% (or about 10%) of the total thickness.

The micrographs also show that for certain foamed polymer samplesfalling above lines 201 and 207 (P>60 g-mm/m²/day) in FIG. 2B (namely,samples #1 through #4), the voids are substantially flattened, notspherical. The flattened shape of the voids in turn causes the polymerbetween the voids to be flattened as well. The flattened shape of thepolymer was found to improve the mechanical properties componentscomprising the foamed polymer. It is believed that having longer lengthsof continuous polymer in the longitudinal direction increases themodulus in this direction. Therefore, at least one embodiment includesthe realization it can be advantageous for the foamed polymer to have atleast some voids, for instance at least 80% or thereabout, that areflattened along the longitudinal axis. The aspect ratio of thisflattening (length to height) is desirably at least 2:1 (or about 2:1)or at least 3:1 (or about 3:1), for example, between 2:1 and 7:1 (orabout 2:1 and 7:1) or between 3:1 and 7:1 (or about 3:1 and 7:1).

It was also observed that for these samples, the voids are not isolatedfrom each other. Many of the voids are connected or joined together.That is, the foamed polymer has “open cells.” The open cellularstructure of these foamed polymer improves breathability, because itallows water vapor to travel a greater distance both axially (ortransversely) and longitudinally, without having to pass through solidpolymer. Desirably, at least 10% (or about 10%) of the voids in a foamedpolymer are interconnected. In some embodiments, at least 20% (or about20%) of the voids are connected to other voids.

TABLE 3 SUMMARY OF VOID FRACTION AND AVERAGE THICKNESS Void AveragePoint # Sample Name Fraction, % thickness, cm FmdAd1 16.8 0.507 1 AB14.2a 33.7 0.0487 2 MB27 4% 41.0 0.0628 3 FIIA-2 46.6 0.173 4 FIIA-553.0 0.198 5 80/20 ARNITEL/PBT 0.0 0.1807 6 80/20 ARNITEL/PBT 20.00.1647 foamed 7 FIIA-1 0.0 0.124 8 TPU/Acetal foamed 10% 15.0-20.0 0.139TPU, ESTANE 58245 0.0 0.048 Batch 15 wts 56.0 0.0799 Batch 15 f 56.00.0799 MB27 0% 0.0 0.0256 MB27 6% 52.0 0.0941 MB22.1 24.1 0.0575 MB32.133.2 0.0448 MB41.4 46.2 0.0829

TABLE 4 SUMMARY OF MODULUS, DIFFUSIVITY, AND PERMEABILITY DiffusivityPermeability, Point Modulus, at RH = g- # Sample Name MPa 97, cm²/secmm/m²/day ARNITEL ® VT 3108 122 2.11 × 10⁻⁷ 36 FmdAd1 96.8 2.30 × 10⁻⁷39 1 AB 14.2 61.5 4.77 × 10⁻⁷ 81.4 2 MB 27 4% 45 5.65 × 10⁻⁷ 96.5 3FIIA-2 47.7 8.66 × 10⁻⁷ 147.6 4 FIIA-5 41.7 13.7 × 10⁻⁷ 234 5 80/20ARNITEL/PBT 375 1.46 × 10⁻⁷ 24.8 6 80/20 ARNITEL/PBT 266  1.9 × 10⁻⁷32.5 foamed 7 FIIA-1 122 2.11 × 10⁻⁷ 36 8 TPU/Acetal foamed 10% 37 6.59× 10⁻⁶ 151 TPU, ESTANE 58245 18 2.41 × 10⁻⁷ 80

In TABLE 4, the permeability data for ARNITEL®-based samples werecalculated using the relation:

$P_{sample} = {P_{{ARNITEL}\mspace{14mu} {VT}\mspace{14mu} 3108}\frac{D_{sample}}{D_{{ARNITEL}\mspace{14mu} {VT}\mspace{14mu} 3108}}}$

where P_(sample) represents the permeability of the sample,P_(ARNITEL VT 3108) represents the permeability of ARNITEL® VT 3108,D_(sample) represents the diffusivity of the sample, andD_(ARNITEL VT 3108) represents the diffusivity of ARNITEL® VT 3108.Similarly, the permeability data for TPU (ESTANE®)-based samples werecalculated using the relation:

$P_{sample} = {P_{{ESTANE}\; 58245}\frac{0.7D_{sample}}{D_{{ESTANE}\; 58245}}}$

P_(ESTANE 58245) and D_(ESTANE 58248) represent the permeability anddiffusivity of ESTANE® 58245, respectively. The factor 0.7 reflects thelower water content of the blended sample.

Another suitable foamed polymer material is polyether-basedthermoplastic polyurethane (TPU), which has good breathability and tearresistance. However, TPU has poor stiffness (a low Young's modulus).Much research has gone into improving the stiffness of the material bymixing it with other polymers. However, it has been found while blendingTPU with other polymers can be effective in increasing stiffness, therecan be a serious decrease in the breathability of the blended polymer.

After testing, blends have been identified which greatly improvemechanical stiffness without reducing the breathability to anunacceptable level. An example blend is the blend of copolyester TPE/PBTdiscussed above. Another example blend comprises TPU andpolycarbonate-acrylonitrile butadiene styrene (PC-ABS, sold asWONDERLOY® for example). A suitable weight ratio of TPU:WONDERLOY® is70:30 (or about 70:30). Tests conducted using a 19 mm diameter singlescrew extruder have shown that tensile strength of the blend exhibits amarked improvement in stiffness over TPU alone (14 fold or thereabout),while the moisture vapour transmission rate shows only a slightreduction in breathability (30% or thereabout). By foaming theTPU-WONDERLOY® polymer blend, a further improvement in the breathabilityversus stiffness can be achieved as described above.

As discussed above, yet another example blend according to at least oneembodiment comprises a TPU (ESTANE® 58245) and acetal, a compound havingvery low breathability (permeability) and water uptake. A foamed strip(void fraction between 15 and 20% or about 15 and 20%) was created fromESTANE® 58245 and acetal in a weight ratio of 70:30 (or about 70:30).The average sample thickness was 0.139 cm. The water uptake of the blendat 100% RH was 0.38 g water per gram dry polymer (38%). The diffusivityof the sample was measured from the desorption curve and found to be6.59×10⁻⁶ cm²/sec at 23° C. The modulus of the sample was 34 MPa, andthe permeability was calculated to be 151 g-mm/m²/day.

These results compare to a control example which comprises unfoamed TPU(ESTANE® 58245). A corrugated tube was extruded having a wall thicknessof 0.048 cm and a water uptake at 100% RH of 0.53 g water per gram ofdry polymer (53%). The diffusivity of the non-foamed sample was measuredfrom the desorption curve and found to be 2.41×10⁻⁷ cm²/s at 23° C. Themodulus was 18 MPa. The permeability of this polymer is 80 g-mm/m²/day.

Components Comprising Foamed Polymers

It will be appreciated that the foamed breathable materials describedabove lend themselves to many medical components where a highlybreathable but self supporting, semi-rigid material is advantageous.Accordingly, all of the particulars of the breathable foamed materialdiscussed above are applicable to these components. The following arejust some examples of components to which the foamed breathable materialprovides new advantages that have previously not been possible.Manipulation of the void fraction, thickness, and void size allows awide range of customisation of the bulk properties of formed components.

In general, a component comprises a wall defining a space within andwherein at least a part of said wall is of a breathable foamed materialas described above, which allows the transmission of water vapor fromgases within the space, but prevents the transmission of liquid water.Preferably, the wall is also impermeable to the bulk flow of gaseswithin the space, including breathing gases, anaesthetic gases,insufflation gases, and/or smoke.

Because of its breathability, the wall forms a water vapor pathway fromthe gases space to the region on the other side of the wall. In someembodiments, there is a water vapor pathway from the gases space toambient air through said breathable foamed material. The pathway throughcan be a direct pathway, and the wall is exposed directly to ambientair. Alternatively, the pathway is indirect, and the pathway passesthrough one or more other walls between the gases space and ambient air.In other configurations, there can be a second gases space (called asweep gases space) on the other side of said wall, instead of ambientair. This sweep gases space can, in turn, vent indirectly to ambientair. In that case, the water vapor pathway runs from the gases space tothe sweep gases space.

In any of the above embodiments, the entire enclosing wall can be formedof the foamed material. In at least one embodiment, at least a region ofthe wall has a thickness between 0.1 and 3.0 mm (or about 0.1 and 3.0mm), such as between 0.1 and 1.5 mm (or about 0.1 and 1.5 mm). Forexample, at least a region of the wall can have a thickness between 0.7and 1.0 mm (or about 0.7 and 1.0 mm) or between 0.7 and 3.0 mm (or about0.7 and 3.0 mm).

In any of the above embodiments, the wall can include at least twozones. The first zone is an outer skin comprising a layer ofsubstantially closed-cell foamed material, and the second zone is aninner layer adjacent the outer layer and between the outer layer and thegases space. The skin thickness can be between 5 and 10% (or about 5 and10%) of the wall thickness, for example, between 10 and 50 μm (or about10 and 50 μm). Each of the first zone and the second zone have voids. Incertain embodiments, no more than 5% (or about 5%) of the voids in thefirst zone exceed a diameter of 100 μm. The voids in the second zone arelarger than the voids in the first zone. For example, in someembodiments no more than 5% (or about 5%) of the voids of said secondzone of foamed material exceed a diameter of 700 μm.

In any of the above embodiments, the wall can also include at least onereinforcing rib stiffening the wall or at least one region where thewall is locally thickened to stiffen the wall.

The component can be a patient interface; or a tube, such as a breathingtube for use in a breathing circuit; or a tube and at least part of apatient interface; or a conduit (that is, a portion of a tube that neednot be closed around its circumference) for use in a breathing circuit;or a mask (including a mask frame and a seal extending around theperimeter of the mask frame, wherein the mask frame comprises the walland a substantial majority of the wall is formed of the breathablefoamed material); or a component of an insufflation system, such as atube or conduit for use in at least part of the exhaust arm of aninsufflation system.

Reference is next made to FIG. 5, which shows a component 501 accordingto at least one embodiment. The component 501 is formed having a wall503, defining a gases space 505 on one side. The wall 503 comprises abreathable foamed polymer as described above. As represented by dottedline 507, the wall may or may not define a completely closed gases space505. When in use, the gases space can be substantially closed so thatthe wall 503 defines gases space 505 on one side of the wall 503 and thespace 505 contains a humid gas.

On the other side of wall 503 is a second gases space 509. In at leastone embodiment, the second gases space 509 is ambient air. The wall 503of component 501 is of a breathable foamed material that allows thetransmission of water vapor but substantially prevents the transmissionof liquid water and the bulk flow of breathing gases. In order for thebreathable foamed material to permit drying of the gases in space 505,the outer surface of the wall 503 is exposed to ambient air or a drysweep gas in a second gases space 509. In such a configuration, gaseshaving a high relative humidity within gases space 505 can be dried bytransmission of water vapor through wall 503 into the second gases space509, which may be for example ambient air. The drying of the gaseswithin gases space 505 is useful to produce and/or prevent rain-outoccurring in the gases space 505 when filled with a relatively warm orhumid gas/air/breathing gas.

In one example, the component 501 may be a patient interface, such as arespiratory mask, and gases space 505 may be at least partially definedby wall 503, and by a patient's face (not shown) to substantiallyenclose the space 505. In this example, the patient's face isrepresented by dotted line 507. In another embodiment, the component 501may be a breathing tube (inspiratory or expiratory). Patient interfacesand breathing tubes are discussed in greater detail below.

Breathable Tubes

In assisted breathing, particularly in medical applications, gaseshaving high levels of relative humidity are supplied and returnedthrough flexible breathing tubes of a relatively restricted size,typically between a range of 10 to 25 mm (or about 10 to 25 mm) diameter(covering both neonatal and adult applications). Such breathing tubesare ideally very light, resistant to kinking or pinching, and flexibleto ensure the greatest performance and level of comfort for the patient.The light weight of a breathing tube is very important to reduce anyforces applied to the patient interface by the weight of the tube.Similarly, breathing tubes must be flexible and able to bend easily toachieve a high level of patient comfort which in turn can improvepatient compliance. However, extremely light and flexible components areusually weak and prone to excessive kinking. It was discovered that atube comprising the above-described foamed polymer can resist kinkingand pinching, yet is light and sufficiently flexible to improve patientcomfort.

Because a tube is a type of component, the particulars of the componentdiscussed above are applicable to the tube discussed here. In general, amedical circuit tube comprises an inlet (for receiving humidifiedgases), an outlet (for expelling humidified gases), and an enclosingwall defining at least one gases passageway between said inlet and saidoutlet, wherein at least a part of said enclosing wall is of abreathable foamed material allowing the transmission of water vapour butsubstantially preventing the transmission of liquid water and the bulkflow of breathing gases. In at least one embodiment, the tube is anextruded corrugated tube. The medical circuit tube can be used as abreathing tube or conduit or a tube or conduit for a limb of aninsufflation system. For instance, the tube can be an expiratorybreathing tube or an exhaust conduit, respectively. The tube can also bepart of a patient interface.

The tube can be flexible. That is, the tube can be bent around a 25 mmdiameter rod without kinking or collapse. More particularly, the tube isflexible as defined by passing the test for increase in flow resistancewith bending according to ISO 5367:2000(E).

In any of the above embodiments, the tube can have a length between 1and 2 m (or about 1 and 2 m), for instance 1.5 m (or about 1.5 m). Atube can have a mean diameter between 10 and 25 mm (or about 10 and 25mm). In at least one embodiment, the tube has a wall thickness between0.1 and 1.2 mm (or about 0.1 and 1.2 mm), for example, between 0.6 mmand 1.0 mm (or about 0.6 and 1.0 mm). Preferably, a tube comprises abreathable enclosing wall over a significant portion of its totallength. For instance, in at least one embodiment, at least 80% of thelength of the tube comprises a breathable enclosing wall. The breathablewall is preferably located proximal the inlet end of the tube forreceiving humidified gas. For example, for a tube 1.5 m (or about 1.5 m)in length, at least 1.2 m (or about 1.2 m) of the tube comprises abreathable wall, starting proximal the inlet end.

Because of its breathability, the wall forms a water vapor pathway fromthe gases space to the region on the other side of the wall. In someembodiments, there is a water vapor pathway from the gases space toambient air through said breathable foamed material. The pathway throughcan be a direct pathway, and the wall is exposed directly to ambientair. For example, in at least one embodiment, the tube is a breathingtube and is terminated by a first connector at said inlet and a secondconnector at said outlet. Only one gases passageway is provided thelength between said inlet connector and said outlet connector.

Alternatively, the pathway is indirect, and the pathway passes throughone or more other walls between the gases space and ambient air. Inother configurations, there can be a second gases space (called a sweepgases space) on the other side of said wall, instead of ambient air.This sweep gases space can, in turn, vent indirectly to ambient air. Inthat case, the water vapor pathway runs from the gases space to thesweep gases space. For example, the tube can be a coaxial breathingtube. In a coaxial breathing tube, the gases space is an inspiratorylimb or an expiratory limb, and the second gases space is the other ofsaid inspiratory limb or an expiratory limb. One gases passageway isprovided between the inlet of said inspiratory limb and the outlet ofsaid inspiratory limb, and one gases passageway is provided between theinlet of said expiratory limb and the outlet of said expiratory limb. Inone embodiment, the gases space is said inspiratory limb, and saidsecond gases space is said expiratory limb. Alternatively, the gasesspace can be the expiratory limb, and the second gases space is theinspiratory limb.

As explained above in conjunction with the description of the component,in any of the above embodiments, the wall can include at least twozones. The first zone is an outer skin comprising a layer ofsubstantially closed-cell foamed material, and the second zone is aninner layer adjacent the outer layer and between the outer layer and thegases space. The skin thickness can be between 5 and 10% (or about 5 and10%) of the wall thickness, for example, between 10 and 50 μm (or about10 and 50 μm). Each of the first zone and the second zone have voids. Incertain embodiments, no more than 5% (or about 5%) of the voids in thefirst zone exceed a diameter of 100 μm. The voids in the second zone arelarger than the voids in the first zone. For example, in someembodiments no more than 5% (or about 5%) of the voids of said secondzone of foamed material exceed a diameter of 700 μm.

Furthermore, in any of the above embodiments, the tube can include aplurality of reinforcing ribs arranged about the enclosing wall. Theseribs can be co-extruded with the tube to be generally aligned with thelongitudinal axis of the tube. Preferably, there are three to eightreinforcing ribs, and more particularly, three to five reinforcing ribs.

In addition to the above, to reduce or eliminate the formation ofcondensation within the tube, and to maintain a substantially uniformtemperature in the gases flow through the tube in use, a heater, such asa resistance heater wire, may be provided within the tube passageway orwithin tube wall.

In a particular embodiment, the tube has a length of 1.525 m (orthereabout), a weight of 54 g (or thereabout), a void fraction of 35%(or thereabout), a pneumatic compliance of 0.23 mL/cm H₂O/m (orthereabout), and a permeability of 85 g-mm/m²/day (or thereabout). Thetube is formed from 95% (or about 95%) ARNITEL® VT 3108 and 5% (or about5%) of a foaming agent masterbatch comprising polyethylene and 20% (orabout 20%) by weight of Clariant HYDROCEROL® BIH-10E.

Reference is next made to FIGS. 6A and 6B, which show a breathable tube601 according to at least one embodiment. FIG. 6A shows a side view ofthe tube 601, while FIG. 6B shows cross-section of the tube 601 alongthe same side view as FIG. 2A. In both FIG. 6A and FIG. 6B, thehorizontal axis is indicated as line 603-603. The tube wall, shown aswall 605 in FIG. 6B is a breathable foamed material, as described above.Wall 605 can be between 100 and 1500 μm (or about 100 and 1500 μm) thickfor a breathing tube of typical dimensions—between 12 and 20 mm (orabout 12 and 20 mm) diameter for neonatal and adult applicationsrespectively and 1 to 2 m (or about 1 to 2 m) in length. However, thewall 605 may be up to 3 mm (or about 3 mm) thick and still deliver goodbreathability.

The tube 601 is corrugated (that is, the tube has a ridged or groovedsurface). The method for forming the corrugated tube is discussed ingreater detail below, with respect to FIG. 15. However, in someembodiments, the tube has a smooth surface.

Reference is next made to FIGS. 7A and 7B, which show a breathable tube701 according to at least one embodiment. Again, the tube 701 ismanufactured from a foamed breathable material, as described in any oneof the examples herein. The tube further includes a plurality ofreinforcing ribs 703 that can be co-extruded with the tube. The form ofthe ribs 703 is determined by the extruder die head, and the size andthe foaming level is controlled by the temperature and pressure when itexits the die head.

The ribs 703 can be formed from the same foamed polymer as the tube 701.Alternatively, the ribs 703 can be made from a different material thanthe tube. This can be achieved by co-extrusion. As shown in FIG. 7A, thetube 701 can be extruded with the ribs 703 in place, and then corrugatedto form the “dotted” structure shown in FIG. 7B. In certain embodiments,a tube includes between three and eight reinforcing ribs, such asbetween three and five reinforcing ribs. Such additionally reinforcedtubes can find independent application in one or more of the tubecomponents described in this specification in relation to medicalcircuits.

Reference is next made to FIGS. 8A and 8B, which show an alternativeconfiguration for a ribbed, breathable tube 801 according to at leastone embodiment. In FIG. 8B, raised ribs 803 are visible in the spacebetween the ridges in the inside of the tube 801. FIG. 8C shows acorrugator suitable for forming the tube shown in FIG. 8A and 8B. Theblock comprises raised portions 805 in between ridge portions 807, whichwill form the raised ribs when the tube is removed from the corrugator.It will be appreciated that still other reinforcing processes may usedto supplement the tube in order to improve its performancecharacteristics still further (such as compliance, pull strength,resistance to flow with bending and crush resistance). Those processesmay or may not be integrated with the tube forming process.

Reference is next made to FIG. 9, which shows another example medicalcircuit according to at least one embodiment. The circuit comprises twobreathable tubes comprising a breathable foamed polymer as describedabove, namely an inspiratory tube 103 and an expiratory tube 117. Theproperties of the inspiratory tube 103 and the expiratory tube 117 aresimilar to the tubes described above with respect to FIG. 1. Theinspiratory tube 103 has an inlet 109, communicating with a humidifier115, and an outlet 113, through which humidified gases are provided tothe patient 101. The expiratory tube 117 also has an inlet 109, whichreceives exhaled humidified gases from the patient, and an outlet 113.As described above with respect to FIG. 1, the outlet 113 of theexpiratory tube 117 can vent exhaled gases to the atmosphere, to theventilator/blower unit 115, to an air scrubber/filter (not shown), or toany other suitable location.

As described above with respect to FIG. 1, heating wires 901 can beplaced within the inspiratory tube 103 and/or the expiratory tube 117 toreduce the risk of rain out in the tubes by raising the temperatureabove the saturation temperature.

In this example, the expiratory tube 117 comprises a connector (here, aY-connector 903) for connecting to other components. For instance, theY-connector 903 is configured to connect to the inspiratory tube 103 anda patient interface (not shown). Of course, the embodiment of FIG. 9 issimply an example configuration. A component according to at least oneembodiment comprises a breathable, foamed polymer tube. The componentcan further comprise a suitable connector. Preferably, the connectoralso comprises the breathable, foamed polymer.

Reference is next made to FIG. 10, which shows a coaxial tube 1001according to at least one embodiment. In this example, the coaxial tube1001 is provided between a patient 101 and a ventilator 1005. Expiratorygases and inspiratory gases each flow in one of the inner tube 1007 orthe space 1009 between the inner tube 1007 and the outer tube 1011. Itwill be appreciated that the outer tube 1011 may not be exactly alignedwith the inner tube 1007. Rather, “coaxial” refers to a tube situatedinside another tube. In use, water vapour, but not liquid water, istransmitted through a foamed, breathable tube wall, as explained below.

For heat transfer reasons, the inner tube 1007 carries the inspiratorygases in the space 1013 therewithin, while the expiratory gases arecarried in the space 1009 between the inner tube 1007 and the outer tube1011. This airflow configuration is indicated by arrows.

The inner tube 1007 is formed using the breathable foamed materialdescribed herein. Thus, humidity in the expiratory flow space 1009 maypass through the foamed breathable material to humidify the inspiratoryflow in inspiratory flow space 1013. With the gases flow in acounter-flow arrangement as shown in the example, the breathablematerial provides substantial passive humidification of the inspiratoryflow.

With a coaxial tube 1001, the ventilator 1005 may not become aware of aleak in the inner tube 1007. Such a leak may short circuit the patient101, meaning that the patient 101 will not be supplied with sufficientoxygen. Such a short circuit may be detected by placement of a sensor atthe patient end of the coaxial tube 1001. This sensor may be located inthe patient end connector 1015. A short circuit closer to the ventilator1005 will lead to continued patient 101 re-breathing of the air volumeclose to the patient 101. This will lead to a rise in the concentrationof carbon dioxide in the inspiratory flow space 1013 close to thepatient 101, which can be detected directly by a CO₂ sensor. Such asensor may comprise any one of a number of such sensors as is currentlycommercially available. Alternatively, this re-breathing may be detectedby monitoring the temperature of the gases at the patient end connector1015, wherein a rise in temperature above a predetermined levelindicates that re-breathing is occurring.

In addition to the above to reduce or eliminate the formation ofcondensation within either the inner tube 1007 or outer tube 1011, andto maintain a substantially uniform temperature in the gases flowthrough the coaxial tube 1001, a heater, such as a resistance heaterwire, may be provided within either the inner tube 1007 or outer tube1011, disposed within the gases spaces 1009 or 1013, or within the innertube 1007 or outer tube 1011 walls themselves.

In an alternative embodiment of a coaxial tube 1001 where passivehumidification is not desired, the foamed breathable wall may be theouter wall of the outer tube 1011. In this arrangement, the outer tube1011 is in contact with ambient air, and the breathable wall allowswater vapor exchange from the relatively humid expiratory gases with theambient air. As a result, rain out can be managed and/or prevented.

Respiratory Mask

In the art of respiration devices, there are a well known variety ofrespiratory masks which cover the nose and/or mouth of a patient inorder to provide a continuous seal around the nasal and/or oral areas ofthe patient's face, such that gas may be provided at positive pressurewithin the mask for consumption by the patient. The uses for such masksrange from high altitude breathing (for example, aviation applications)to mining and fire fighting applications, to various medical diagnosticand therapeutic applications.

One application of such a mask is in respiratory humidificationtreatment. This system normally consists of a ventilator, humidifier,breathing circuit and patient interface, such as a mask or nasalcannula. In this form of treatment, humid air is supplied to the patientand as a result of the temperature difference between the humid air andthe surrounding environment, the humid air can condense and form waterdroplets. In cases where treatment is prolonged (up to several days)these droplets may form water pools in the mask that can hamper thetreatment, increase the risk of the patient inadvertently inhaling waterand may cause discomfort and/or choking to the patient.

One requisite of such respiratory masks has been that they provide aneffective seal against the patient's face to prevent leakage of the gasbeing supplied. Commonly, in prior mask configurations, a goodmask-to-face seal has been attained in many instances only withconsiderable discomfort for the patient. This problem is most crucial inthose applications, especially medical applications, which require thepatient to wear such a mask continuously for hours or perhaps even days.In such situations, the patient will not tolerate the mask for longdurations and optimum therapeutic or diagnostic objectives thus will notbe achieved, or will be achieved with great difficulty and considerablepatient discomfort.

Described below are various improvements in the delivery of respiratorytherapy. In particular, a patient interface is described which iscomfortable for the patient to wear and includes at least in part awater vapor permeable (breathable) area in the body of the patientinterface made of a foamed breathable material as described herein. Alarge portion of the mask body (or the entire mask body) can be made ofthe foamed breathable material, taking advantage of the unique strengthproperties and high breathability.

Reference is next made to FIG. 11A and 11B, which show a respiratorymask 1101 according to at least one embodiment. It will be appreciatedthat this patient interface can be used in respiratory care generally orwith a ventilator but will now be described below with reference to usein a humidified Positive Airway Pressure (PAP) system. It will also beappreciated that the following description can be applied to nasalmasks, oral masks, oronasal masks, nasal prongs, and full-face masks dueto the materials ability to be formed into a self-supporting, semi-rigid structure with high breathability, rather than being limited tothe very thin film structures of the prior art.

The mask 1101 includes a hollow body 1103 with an inlet 1105 forconnection to an inspiratory breathing tube. The mask 1101 is positionedon the face of the patient 101 with the headgear 1109 secured around theback of the head of the patient 101. The restraining force from theheadgear 1109 on the hollow body 1103 ensures enough compressive forceon the mask cushion 1111, to provide an effective seal against thepatient 101 face. A number of engaging clips are connected to the bodyfor the attachment of sliding members to connect the mask 1101 to theheadgear 1109. The expiratory gases can be expelled through a valve (notshown) in the mask 1101, a further expiratory conduit (not shown), orany other such method as is known in the art.

The hollow body 1103 is constructed of a foamed polymer material asdescribed herein. Such a material provides the requisite rigidity to themask 1101 as well as being highly breathable. Previous attempts toprovide a mask 1101 with breathable areas have required the use of thinmembranes in order to achieve sufficiently high breathability. Thesemembranes have had to be supported by additional reinforcing such as astrong mask frame and have also needed protection from damage. The areasof breathable membrane are usually supported within cutout areas of themask frame. However, with the self-supporting breathable foamed polymersdescribed herein, large portions of the mask 1101 (or the entire mask1101) can be made of the foamed polymer, taking advantage of the uniquestrength properties and high breathability. The result is a selfsupporting semi-rigid mask 1101 that can be entirely (and highly)breathable.

Alternatively, the hollow body 1103 could have large areas cut out ofthe front surface such that the hollow body 1103 substantially consistsof a framework having an outer circumference. Inserts made from thefoamed, self-supporting breathable material described herein can beplaced in the cut outs and bonded in order to prevent or reduce theformation of water droplets inside the mask 1101 during prolongedhumidification treatment, thereby allowing moisture to escape to thesurrounding ambient environment. A number of techniques exist as a meansof attaching the breathable structure to the hollow body 1103 which mayinclude gluing, sonic welding techniques, over-molding co-extrusion, ora snap-tight connection between the foamed breathable insert and thehollow body 1103.

It will be appreciated that additional reinforcing structures may alsobe provided, for example to a mask made of the foamed breathablematerial, to further customise the flexural properties of the component.For example, ribs may be added to the interior and/or exterior surfaceof the mask. Local variations of the wall thickness may also be employedto stiffen/weaken some areas to improve fitting to a patient facialfeatures and/or to provide regions of even greater breathability. Inparticular, this type of reinforcement can be very useful to tailor theflexural properties of a component in particular directions wheredifferent loading patterns are anticipated. These advantages have notbeen possible or as easy to achieve with very thin breathable membranesused previously.

Nasal Cannula

Reference is next made to FIG. 12, which shows a nasal cannula 1201patient interface according to at least one embodiment. The nasalcannula 1201 comprises a cannula body 1203 and a short delivery tube1205. The foamed breathable polymer described herein can be utilized inthe cannula body 1203 and/or the short delivery tube 1205 to manageand/or prevent rainout from occurring within the gases spaces of thesecomponents. As also described earlier, application can also be found inthe inspiratory breathing tube 601.

Catheter Mount

Yet another medical circuit component to which breathable foamedpolymers can be applied is catheter mounts. A catheter mount connectsbetween a patient interface component such as a mouth piece, nasal mask,or endotracheal tube and the dual limbs or breathing tubes of abreathing circuit. Connection with the dual limbs of the breathingcircuit is generally via a Y-connector. In the patient inhalation andexhalation cycles, the dual limbs of the breathing circuit each have adistinct role: one as inhalation conduit and one as exhalation conduit.The catheter mount serves a dual role, transporting both inhaled andexhaled gases. Accordingly, the catheter mount can have significantdisadvantages. A volume of exhaled air remains in the catheter mountbetween exhalation and inhalation. Accordingly some air is re-breathedby the patient. While not unacceptable, re-breathing is not generallydesirable and where significant re-breathing is likely, a boost inoxygen supply levels may be required.

Gases inhaled by a patient are, in a well managed ventilation system,delivered in a condition having humidity near a saturation level and atclose to body temperature, usually at a temperature between 33 and 37°C. (or about 33 and 37° C.). This temperature may be maintained by aheater in the inhalation breathing tube right up to the point where thegases enter the catheter mount. Gases exhaled by a patient are returnedfully saturated and are subjected to further cooling as they flowthrough the catheter mount. Accordingly, although little condensationforms on the interior walls during patient inhalation, significantcondensation levels may form during patient exhalation. Thecondensation, or rain out, occurring inside the catheter mount isparticularly deleterious due to its proximity to the patient. Mobilecondensate breathed or inhaled by a patient may lead to coughing fits orother discomfort.

Reference is next made to FIG. 13, which shows a catheter mount 1301according to at least one embodiment. The catheter mount 1301incorporates a Y-connector 1303 at the ventilator end. An inner tube1305 extends coaxially with an outer tube 1307. The inner tube 1305 issupported at its patient end by an inner tube connector 1309, which inturn is supported via support struts 1311 from patient end connector1313. The inner tube 1305 is supported at its other end by a secondinner tube connector 1315, which forms part of the ventilator endY-connector 1303.

The second inner tube connector 1315 communicates with the inspiratorybreathing tube connector 1317. The outer tube 1307 has at least a partof its wall being made from a foamed breathable material as describedherein. In certain embodiments, the outer tube 1307 is formed entirelyfrom foamed breathable material.

Therefore, in use, the catheter mount 1301 has an inspiratory flow 1319entering the catheter mount 1301. The inspiratory flow 1319 passesthrough the inner tube 1305 to exit to the patient through the patientend connector 1313. Upon patient exhalation, whether assisted orotherwise, expired gases 1321 pass through patient end connector 1313and into the space surrounding the inner tube 1305. These expired gases1321 pass along the inside of the wall of outer tube 1307 and outthrough the expiratory breathing tube connector 1323 of the Y-connector1303. In passing through the catheter mount 1301 within the spacebetween the inner tube 1305 and the outer tube 1307, water vapor maypass through the water vapour permeable foamed outer tube 1307. Incertain embodiments, the entire of outer tube 1307 is breathable. Inthis way, although the expired gases 1321 may experience sometemperature drop as they pass through the catheter mount 1301 to theexpiratory breathing tube connector 1323, hand in hand with thistemperature drop is a reduction in humidity by water vapor passingthrough the breathable foamed material of the outer tube 1307.Accordingly, relative saturation of the expiratory flow is reduced andrain out is thereby also reduced. The tube walls made of the foamedbreathable material can have a wall thickness between 0.1 and 3 mm (orabout 0.1 and 3.0 mm) and be sufficiently stiff to be self supporting orsemi-rigid while still maintaining a high breathability.

The catheter mount 1301 incorporating the breathable foamed polymersdescribed herein includes explicit division of the inspiratory andexpiratory flows through the catheter mount 1301, thereby significantlyreducing re-breathing. Rain out is also reduced by reducing the humidityof the expired gases even as the temperature of those gases reduces.

Component of an Insufflation or Smoke Evacuation System

Laparoscopic surgery, also called minimally invasive surgery (MIS), orkeyhole surgery, is a modern surgical technique in which operations inthe abdomen are performed through small incisions (usually 0.5 to 1.5cm) as compared to larger incisions needed in traditional surgicalprocedures. Laparoscopic surgery includes operations within theabdominal or pelvic cavities.

During laparoscopic surgery with insufflation, it may also be desirablefor the insufflation gas (commonly CO₂) to be humidified before beingpassed into the abdominal cavity. This can help prevent “drying out” ofthe patient's internal organs, and can decrease the amount of timeneeded for recovery from surgery. Even when dry insufflation gas isemployed, the gas can become saturated as it picks up moisture from thepatient's body cavity. The moisture in the gases tends to condense outonto the walls of the discharge limb or conduit of the insufflationsystem. The water vapour can also condense on other components of theinsufflation system such as filters. Any vapour condensing on the filterand run-off along the limbs (inlet or exhaust) from moisture is highlyundesirable. For example water which has condensed on the walls, cansaturate the filter and cause it to become blocked. This potentiallycauses an increase in back pressure and hinders the ability of thesystem to clear smoke. Further, liquid water in the limbs can run intoother connected equipment which is undesirable.

In abdominal surgery, for example, the abdomen is usually insufflatedwith carbon dioxide gas to create a working and viewing space. The gasused is generally CO₂ which is common to the human body and can beabsorbed by tissue and removed by the respiratory system. It is alsonon-flammable, which is important because electrosurgical devices arecommonly used in laparoscopic procedures. It has been common practice inlaparoscopic surgery to use dry gases. However, it is also desirable forthe CO₂ or other insufflation gas to be humidified before they arepassed into the abdominal cavity. This can help prevent ‘drying out’ ofthe patient's internal organs, and can decrease the amount of timeneeded for recovery from surgery. Insufflation systems generallycomprise humidifier chambers that hold a quantity of water within them.The humidifier generally includes a heater plate that heats the water tocreate a water vapour that is transmitted into the incoming gases tohumidify the gases. The gases are transported out of the humidifier withthe water vapor.

Surgical procedures frequently involve electrosurgery or electrocauteryor increasingly the use of lasers. The use of these devices tends tocreate surgical smoke in the working space due to burning of tissue.Smoke evacuation systems which use a discharge arm or limb are commonlyused to remove the smoke from the surgical site, so that a surgeon cansee what he or she is doing, and so that this potentially harmfulmaterial does not remain within the body cavity post-surgery. One end ofthe discharge arm or limb is connected to, or inserted into, a secondincision (or sometimes the same incision). A typical smoke evacuationsystem generally includes a trocar and a cannula at the end to aidinsertion into the operative site. The smoke exits the insufflatedabdominal area through the discharge limb. The discharge limb may beattached to the end of a laparoscopic instrument so as to provideevacuation close to the site where electrocautery takes place. Usually,the gases and smoke from the body cavity are filtered through a filterto remove particulate matter before they are vented to atmosphere. Thefilter may also be additionally designed to remove chemicals and anyharmful micro-organisms from the surgical smoke.

Reference is next made to FIG. 14, which shows an insufflation system1401, according to at least one embodiment. The insufflation system 1401includes an insufflator 1403 that produces a stream of insufflationgases at a pressure above atmospheric for delivery into the patient 1405abdominal or peritoneal cavity. The gases pass into a humidifier 1407,including a heater base 1409 and humidifier chamber 1411, with thechamber 1411 in use in contact with the heater base 1409 so that theheater base 1409 provides heat to the chamber 1411. In the humidifier1407, the insufflation gases are passed through the chamber 1411 so thatthey become humidified to an appropriate level of moisture.

The system 1401 includes a delivery conduit 1413 that connects betweenthe humidifier chamber 1411 and the patient 1405 peritoneal cavity orsurgical site. The conduit 1413 has a first end and second end, thefirst end being connected to the outlet of the humidifier chamber 1411and receiving humidified gases from the chamber 1411. The second end ofthe conduit 1413 is placed in the patient 1405 surgical site orperitoneal cavity and humidified insufflation gases travel from thechamber 1411, through the conduit 1413 and into the surgical site toinsufflate and expand the surgical site or peritoneal cavity. The systemalso includes a controller (not shown) that regulates the amount ofhumidity supplied to the gases by controlling the power supplied to theheater base 1409. The controller can also be used to monitor water inthe humidifier chamber 1411. A smoke evacuation system 1415 is shownleading out of the body cavity of the patient 1405.

The smoke evacuation system 1415 can be used in conjunction with theinsufflation system 1401 described above or may be used with othersuitable insufflation systems. The smoke evacuation system 1415comprises a discharge or exhaust limb 1417, a discharge assembly 1419,and a filter 1421. The discharge limb 1417 connects between the filter1421 and the discharge assembly 1419, which in use is located in oradjacent to the patient 1405 surgical site or peritoneal cavity. Thedischarge limb 1417 is a self-supporting tube (that is, the tube iscapable of supporting its own weight without collapsing) with two openends: an operative site end and an outlet end.

The gases supplied by the insufflation system 1401 are alreadyhumidified at the point of entry to the patient 1405 body cavity. As thebody cavity is already moist and humid, the gases do not tend to losemoisture in the body, and can become fully saturated if they are notalready at saturation point. If the gases are dry on entry to the bodycavity, they tend to become humidified as they pass through the bodycavity, picking up moisture from the damp atmosphere in the body cavityabove the internal organs.

When these saturated gases pass out of the patient 1405 body cavity,they pass along the cooler walls of the discharge limb 1417, which isnormally 1 m (or thereabout) in length. The moisture in the gases tendsto condense out of the gas onto the walls of the discharge limb 1417,discharge assembly 1419, and/or the filter 1421. The vapor condensing onthe filter 1421 and run-off along the discharge limb 1417 from moisturewhich has condensed on the walls, can saturate the filter 1421 and causeit to become blocked. This potentially causes an increase in backpressure and hinders the ability of the system to clear smoke.

The condensed moisture within the filter 1421 can cause the filter 1421to become partially or totally blocked, leading to an increase in backpressure and reduced filter efficiency due to the blockage. This isdisadvantageous because the increased back pressure hinders the abilityof the system to effectively clear the surgical smoke. The surgicalsmoke remaining at the operational site within the surgical cavity orwithin the conduit of the evacuation system can be hazardous to thepatient since the surgical smoke contains several potential toxins thatmay become entrained in the surgical cavity or tissue of the patient1405. The vision of the surgeons can be obstructed or hindered due tothe surgical smoke remaining at the operational site and not beingevacuated, potentially leading to a hazardous working environment forthe surgeons. The condensation may partially block the filter 1421resulting in reduced filtration of toxins from the surgical smoke. Thiscould result in potentially harmful substances like odors, surgicalsmoke, dead cellular matter, and so on escaping into the operatingtheater. These sorts of materials can be hazardous to the health and maylead to many health problems for medical practitioners and the patient.

At least one embodiment includes the realization that the use of adischarge limb 1417 having a breathable wall or the wall of the limbwhich includes breathable material will help to alleviate this problem.In particular, a foamed breathable material as described herein isespecially suitable for forming this type of discharge limb 1417 conduitof an insufflation system because of the properties discussed inrelation to foamed material, component, and breathing tube describedearlier. A certain amount of moisture from the expelled gases passesthrough the wall of the discharge limb 1417 before reaching the filter1421, and therefore there is less moisture in the gas to condense out ofthe gas and clog the filter 1421. Accordingly, the discharge limb 1417is preferably made of a breathable, foamed material as described herein.The process detailed below for manufacturing a breathing tube can beapplied directly to insufflation system tubes, including inlet orexhaust (smoke evacuation) limbs.

Method of Manufacture

Reference is next made to FIG. 15, which illustrates an example methodof manufacturing a breathable component suitable for deliveringhumidified gas, such as a tube as in FIG. 2A and FIG. 2B or any othertube discussed herein, according to at least one embodiment.

In general, a method of manufacturing a component involves mixing afoaming agent into a polymeric base material and forming a liquefiedmixture. The foaming agent is allowed to release gas bubbles into thebase material portion of the liquefied mixture. Then, the release of gasbubbles is arrested and the mixture is solidified to form the desiredcomponent. Desired properties of the finished component are discussedabove.

In at least one embodiment, the process used to make a component such asa breathing tube involves extruding a molten extrudate 1501 into acorrugator 1503 to form the desired component, such as a tube 1505. Incertain embodiments, the polymeric base material for the extrudate has adiffusion co-efficient greater than 0.75×10⁻⁷ cm²/s (or thereabout). Thebase material can have the following stiffness properties: (a) a tensilemodulus greater than 15 MPa (or about 15 MPa), which can be desirablefor urethane-thermoplastic-elastomer-based base materials (or TPU-basedbase materials, as defined by ISO 18064:2003(E)); or (b) a tensilemodulus greater than 100 MPa (or about 100 MPa), which can be desirablefor copolyester-thermoplastic-elastomer-based base materials (orTPC-based base materials, as defined by 18064:2003(E)), such asARNITEL®-based base materials. These foregoing properties are simply byway of example. A base material need not have these properties toproduce a foamed material with the desired breathability and stiffness,and the example modulus numbers are not expressly limited to TPU andTPC-based base materials.

An extruder such as a Welex extruder equipped with a 30 mm diameterscrew and a 12 mm annular die head with a gap of 0.5 mm has been foundto be suitable producing low cost tubes quickly. After exiting theextruder die head 1507, the molten tube 1501 can be passed between aseries of rotating molds or blocks on the corrugator 1503. A corrugatorsuch as those manufactured and supplied by UNICOR® also has been foundto be suitable. This forms a corrugated tube 1505.

The above-described method is simply by way of example. Alternatemethods for forming components comprising the foamed materials describedherein are also suitable. For instance, another method for manufacturinga breathable component involves extruding a strip of foamed material,winding the foamed strip on a mandrel, and sealing the seams of thewound strip with a bead (such as a bead of the foamed material).

Foaming during the extrusion process can be done in several ways,including physical foaming and chemical foaming

In physical foaming, the foaming agent is an inert gas (e.g. CO₂ or N₂),which is injected in the extruder barrel at a flow rate and a pressuresufficiently high to dissolve it into the molten polymer. For example, apressure greater than 100 bar (or about 100 bar) and a flow rate aslittle as 1% (or about 1%) of the polymer flow rate may be suitable.Preferably, a nucleating agent is also introduced into the polymer tocreate sites for the foam bubbles to expand. An example of this methodincludes using a commercial unit by Sulzer to inject inert gasses at theend of the extruder barrel and mixing the gas with static mixers priorto die exit.

Chemical foaming involves the addition of a chemical that induces achemical decomposition reaction (endothermic or exothermic) when heated,thereby releasing gases. The gases dissolve into the polymer melt duringthe extrusion process due to the pressure in the melt being higher thanthe gases' critical solubilization pressure. The gases come out ofsolution when encountering a pressure drop, such as that at (or soonafter) the die head exit. Foaming agents act as plasticizers, therebyreducing the viscosity of the melt. A reduction in viscosity translatesto a reduced melt pressure for a given temperature, shear rate, and diehead geometry. Accordingly, care should be taken to ensure that thegases do not foam prematurely by keeping the pressure in the extruderabove the critical solubilization pressure. This pressure can bemaintained by controlling the shear rate at the die head and/or thetemperature of the melt.

An example process suitable for foaming the material in the extruderprior to corrugating the tubes involves adding a chemical foaming agentin amounts of 0.3 to 1.5% (or about 0.3 to 1.5%) by weight to a basepolymer (such as ARNITEL® VT 3108). This can be achieved by directlymixing a foaming agent powder (such as HYDROCEROL® CT 671 or anequivalent) with the base polymer or by first mixing a foaming agent“masterbatch” (that is, a mixture of a carrier polymer, such aspolyethylene, and active foaming agent (such as HYDROCEROL® BIH-10E oran equivalent) at 80/20% or about 80/20% by weight of carrier polymer toactive foaming agent before feeding the mixture into the feed zone ofthe extruder barrel. In the first case, the foaming agent powder is thefoaming agent. In the second case, the foaming agent masterbatch is thefoaming agent. HYDROCEROL® CT 671 has a decomposition temperature of160° C. and a solubilization pressure of 60 bar. ARNITEL® VT 3108 has amelt temperature of 185° C. Therefore, in this extrusion example, theprocessing temperatures can be dropped by 10 to 20° C. (or about 10 to20° C.) to prevent the pressure from dropping below the critical value,because dropping melt temperature increases viscosity.

The shear rates (via extruder speed) are set high enough to ensure thatthe pressure is higher than the critical pressure as well as to ensurethat the foaming agent is mixed well with the molten polymer. Once thepolymer exits the die head, foaming starts to take place and bubbles canbe seen to nucleate, and expand until the polymer is cooled to a pointwhere the forces of bubble expansion are lower than the forces requiredto deform the molten polymer (for example, below the melting temperatureof the polymer or below the activation temperature of the foamingagents, where the foaming reaction starts/stops). Cooling begins whenthe polymer enters the corrugator and is molded onto the corrugatorblocks. Those blocks, in turn, are cooled by the corrugator water supplyand the forming vacuum.

Once foamed, the component consists of a corrugated tube havingthousands of foamed cell voids distributed throughout the thickness ofthe component wall. It has been found that for a typical breathing tubecomponent, a void size diameter not exceeding about 700 μm (95%confidence level) in the transverse direction can produce a desirableproduct. However, it is advantageous that the void size diameter in thetransverse direction is smaller than 700 μm to prevent voids extendingentirely through the thickness of the tube wall and causing a leak path.For example, in some embodiments the void size diameter in thetransverse direction may not exceed about 500 μm (95% confidence level).It has also been found that a void size diameter in the transversedirection between 75 and 300 μm (or about 75 and 300 μm) produces a highquality product for medical circuits. The maximum void size diameter inthe transverse direction will depend on the minimum wall thickness ofthe component. For example, the maximum void size in the transversedirection can be limited to less than half (or about half) of theminimum wall thickness. However, the maximum void size diameter in thetransverse direction can be less than one third (or about one third),less than 30% (or about 30%), or even less than one quarter (or aboutone quarter) of the minimum wall thickness.

As discussed above, the foaming bubbles stop growing as the materialcools. It has been found that quickly cooling results in the formationof two zones through the thickness of the wall. FIGS. 16A and 16B showan extruded foamed material comprising two zones, according to at leastone embodiment. A first zone 1601, 100 μm thick (or about 100 μm thick),forms as an outer “skin” of closed cell foamed material on the surfacethat contacts the corrugator molds/blocks. In this zone, the average andmaximum void size is smaller and the skin is less likely to form a leakpath through the wall. In the remaining second zone 1603, the materialcools slower and larger voids can result in open cells. Accordingly, atleast one embodiment includes the realization that it is desirable tocool the material quickly after foaming has initiated as it exits thedie head.

The tube is cooled as part of the corrugation process once it comes intocontact with the corrugator blocks (the metal blocks where the profileshape is machined into). Quick cooling is achieved by maintaining thecorrugator blocks temperature to a low value, for instance, 15° C. (orabout 15° C.), using a coolant such as water. Quick cooling may also beachieved by modifying the melt temperature at the exit of the extruder(and before touching the blocks) to a temperature near the melting pointof the polymer so that the molten plastic solidifies quickly. This canbe accomplished with an air gap between the extruder and corrugator,which can be augmented with cooling gases and/or air jets or a liquidbath, such as a water bath. Quick cooling can also achieved byincreasing the vacuum pressure in the blocks so that the polymer gets“sucked” into the metal shape very quickly and thus cools down beforethe bubbles have time to expand fully. One or more of these techniquescan be used alone or in combination to achieve quick cooling in variousembodiments.

The formation of the skin depends not only on quick cooling. Skinformation also depends on the material composition (such as the level offoaming), the extruder speed, the melt temperature and pressure, the gapbefore cooling, the water temperature and length of the bath, andfinally the haul off speed (a mechanism that pulls the formed tube fromthe extruder). Quick cooling depends mainly on the haul off speed, thegap, and the water temperature.

The resulting skin thickness can be between 5 and 10% (or about 5 and10%) of the wall thickness, for example, between 10 and 50 μm (or about10 and 50 μm). Each of the first zone and the second zone have voids. Incertain embodiments, no more than 5% (or about 5%) of the voids in thefirst zone exceed a diameter of 100 μm. The voids in the second zone arelarger than the voids in the first zone. For example, in someembodiments no more than 5% (or about 5%) of the voids of said secondzone of foamed material exceed a diameter of 700 μm.

Reference is next made to FIG. 17, which describes an example method ofmanufacturing a tube according to at least one embodiment. In theexample method, a foaming agent is first mixed into a base material toform an extrudate, as shown in block 1701. The base material comprisesone or more breathable thermoplastic elastomers having a diffusioncoefficient greater than 0.75×10⁻⁷ cm²/s (or about 0.75×10⁻⁷ cm²/s) anda tensile modulus greater than about 15 MPa. Pressure is then applied tothe extrudate using an extruder to form a hollow tube, as shown in block1703. The hollow tube is delivered to a corrugator mold, as shown inblock 1705. The hollow tube is allowed to cool within the corrugatormold, thereby allowing the foaming agent portion of the extrudate torelease gas bubbles, as shown in block 1707. Finally, the cooled hollowtube is removed from the corrugator, as shown in block 1709, therebyforming a tube comprising solid thermoplastic elastomer and voids formedby the gas bubbles. In this example, the resulting tube has a wallthickness between 0.1 and 3.0 mm (or about 0.1 and 3.0 mm). And themaximum void size is less than one third (or about one third) of theminimum wall thickness and the void fraction of the corrugated tube isgreater than 25% (or about 25%).

Measurements

Properties, including modulus, void fraction, mass, diameter, thickness,and diffusivity, are referred to above. The following provides preferredmethods for measuring these properties. All measurements are at roomtemperature (23° C. or thereabout).

A. Modulus

Tensile measurements were carried out to determine the force/strainrelationship of foamed corrugated tubes under constant extension. It wasdetermined that this relation is typically linear for up to 10%extension. An Instron machine equipped with a 500N load cell was used tocarry out this experiment and 200 mm long corrugated tube samples wereused as the test specimens.

A 2D axi-symmetric finite element (numerical) model was implemented toextract the material's Young's modulus from the experiment. The geometryin this model was constructed from measurements of the corrugated tubes.The model included a linearly elastic (Hookean) material behavior forthe analysis of a modulus of (E). The use of linear elastic materials inthe model is justified under the small extension conditions. The modelwas constrained from one end and pulled from the other with a constantload to simulate a similar behaviour to that seen on the Instronmachine. Extension values at different moduli (E) were extracted fromthe model and the model data was compared to those in the Instronexperiment according to the following equality:

$\lbrack \frac{FL}{ɛ} \rbrack_{model} = \lbrack \frac{FL}{ɛ} \rbrack_{Instron}$

where

F represents force

L represents specimen length, and

ε represents extension.

The modulus was chosen as the value that matches this equality betweenthe model and the experiment. A validation experiment was carried outusing a corrugated tube with a known modulus and the results agreed wellwith the numerical model.

B. Void Fraction Measurement

The void fraction (4) of the foamed polymer sample is defined in EQ. 1as:

$\begin{matrix}{\varphi_{v} = {1 - \lbrack \frac{\rho (S)}{\rho (P)} \rbrack}} & (1)\end{matrix}$

where ρ(S) is the density of a foamed polymer sample and ρ(P) is thedensity of the corresponding unfoamed polymer. Two example methods formeasuring ρ(S) are the buoyancy method and the displacement method,discussed below.

The buoyancy method involves measuring the mass of a sample suspended inair (M₁) and then measuring the mass of the sample suspended in a fluidof a known, low density (M₂), such as heptane. The density of the foamedpolymer sample can be calculated according to EQ. 2 as follows:

$\begin{matrix}{{\rho (S)} = \frac{M_{1}\rho_{F}}{M_{1}M_{2}}} & (2)\end{matrix}$

where ρ_(F) represents the density of the suspension fluid. The buoyancymethod is suitable for smaller samples, when the density of the sampleis larger than the density of the suspension fluid. For example, ifheptane is employed as the suspension fluid, this method is suitable forfoamed ARNITEL® samples having a void fraction less than 45%.

The displacement method involves calculating the volume of a sample bymeasuring the amount of liquid that it displaces. Using a digital heightmeter, the height of the markings on an empty graduated cylinder aremeasured. This provides a calibrated correlation between height andvolume. A liquid is placed in the cylinder and, measuring to the bottomof a concave meniscus or to the top of a convex meniscus, the height ofthe liquid in the cylinder is determined. This gives an initial volume(V₁). Then a sample of foamed polymer of known dry mass (M₁) is placedin the liquid and the height of the liquid in the cylinder is againdetermined. This gives a final volume (V₂). The density of the foamedpolymer sample can be calculated according to EQ. 3 as follows:

$\begin{matrix}{{\rho (S)} = \frac{M_{1}}{V_{2} - V_{1}}} & (3)\end{matrix}$

While the displacement method requires a larger sample to get adequateprecision, it does allow for measurement of lower density samples,because a sample can be submerged and held in place.

C. Mass

All masses were obtained using a Vibra AJ-420 CE tuning forkmicrobalance manufactured by Shinko Denshi Co. (Plant #504068).

D. Thickness and Diameter

Sample thicknesses and/or diameters can be obtained in the followingfashions.

For tubular samples, a Mitutoyo digital calliper (Model CD-8 CSX) can beused to measure diameter. Sample diameters can be measured at multiplepoints and a simple average of these measurements taken as the samplediameter.

For film samples, thicknesses can be obtained at many points using aMitutoyo vernier micrometer D(0-25 mm) RH NEO MODELSHOP. Again, a simpleaverage can be taken as the sample thickness.

For measuring thickness of a corrugated tubular sample, the tube can becut into sections and numerous measurements taken using the digitalcalliper at various positions along the profile. An area-weightedaverage thickness can be calculated. In the alternative, a calibratedmicroscope, such as a Meiju Techno Microscope, can also be used tomeasure the thickness of a corrugated tubular sample. The methodinvolves taking many measurements (typically more than 90) of both thepeak and valley thicknesses along the length of the tube at differentpositions around the circumference. This is accomplished by cutting thetube in half, but along a helical path that covers 45 corrugations perhelical turn.

E. Diffusivity

The time dependent sorption and desorption of water by polymeric systemsis a function of the diffusivity of water in the polymer. Crank J. Themathematics of diffusion. 2nd ed. Oxford: Clarendon Press; 1975 givesdetailed descriptions of how experimental data can be analysed to yieldthe diffusion coefficient of water in a polymer. Pages 46-49, 60, 61,and 72-75 of Crank are hereby incorporated by this reference.

According to Crank, when the diffusion coefficient D is a constant, thedesorption/absorption of water in a sample of thickness 2l is defined byEQ. 4.

$\begin{matrix}{\frac{M(t)}{M(\infty)} = {1 - {\sum\limits_{n}\; {A_{n}{\exp ( {{- \beta_{n}^{2}}{Dt}} )}}}}} & (4)\end{matrix}$

where:

$\frac{M(t)}{M(\infty)}$

represents fractional mass loss or mass gain

M(t)=m(t)−m(0), grams

M(∞)=m(∞)−m(0), grams

m(0) represents mass at time=0, grams

m(t) represents mass at time=t, grams

m(∞) represents mass of the sample at very long times, grams

n represents the nth term in the infinite sum

$\begin{matrix}{A_{n} = \frac{8}{( {{2n} + 1} )^{2}\pi^{2}}} & (5) \\{\beta_{n} = \frac{( {{2n} + 1} )\pi}{2l}} & (6)\end{matrix}$

D represents the diffusion coefficient, cm²/sec

t represents time, sec, and

2l represents the thickness of the sample, cm.

In EQ. 4, the leading exponential for n=1 (that is, with A₁ and β₁)becomes the dominant term for values of

$\frac{M(t)}{M(\infty)} > {0.4.}$

An alternative representation for

$\frac{M(t)}{M(\infty)}$

is obtained by solving the diffusion equation using Laplace transforms.The result is given by Crank and reproduced below as EQ. 7.

$\begin{matrix}{\frac{M(t)}{M(\infty)} = {2\sqrt{\frac{Dt}{l^{2}}}\{ {\sqrt{\frac{1}{\pi}} + {2{\sum\limits_{n}\; {( {- 1} )^{n}i\; {{erfc}( \gamma_{n} )}}}}} \}}} & (7)\end{matrix}$

where

$\gamma_{n} = \frac{nl}{\sqrt{Dt}}$${{{i{erfc}}(x)} = {\lbrack \frac{\exp ( {- x^{2}} )}{\sqrt{\pi}} \rbrack - {{x{erfc}}(x)}}},$

and

erfc(x) represents the complementary error function of x.

At short times, for values of

${\frac{M(t)}{M(\infty)} > 0.4},$

only the term

$\sqrt{\frac{1}{\pi}}$

inside the curly braces {−} contributes most significantly. EQ. 7suggests that plots of

$\frac{M(t)}{M(\infty)}$

versus √{square root over (t)} will be straight lines with a slope equalto

$\frac{2}{l}{\sqrt{\frac{D}{\pi}}.}$

FIG. 18 shows an idealized sorption/desorption curve with a constantdiffusion coefficient D=3.0×10⁻⁷ cm²/sec and l=0.075 cm. Actualexperimental curves, as shown in FIG. 19, look different than theidealized curve. Compared with the idealized curve, the fractional masschange in the experimental curves look retarded in time, and the overallexperimental curves have a sigmoid shape. A sigmoid shape is obtainedwhen desorption of water from the film is limited by the rate ofevaporation at the surfaces of the film. This is mathematicallydescribed by the boundary condition in EQ. 8 at the surface of thematerial.

$\begin{matrix}{{{- D}\frac{\partial C}{\partial x}} = {\alpha ( {C_{0} - C_{s}} )}} & (8)\end{matrix}$

where

-   -   C₀ represents the concentration in the film that would be in        equilibrium with the external environment, g/cm³    -   C_(s) represents the concentration of water just inside the        surface, g/cm³, and    -   α represents a constant related to the rate of evaporation at        the surface, cm/sec.

With evaporation, the analog to EQ. 1 can be expressed according to EQ.9.

$\begin{matrix}{\frac{M(t)}{M(\infty)} = {1 - {\sum\limits_{n}{A_{n}{\exp ( \frac{{- \beta_{n}^{2}}{Dt}}{l^{2}} )}}}}} & (9)\end{matrix}$

where

$\begin{matrix}{{A_{n} = \frac{2L^{2}}{\beta_{n}^{2}( {\beta_{n}^{2} + L^{2} + L} )}}{L = \frac{l\alpha}{D}}} & (10)\end{matrix}$

and β_(n) is a solution to the equation L=β_(n) tan(β_(n)) (11)

Again, at longer times, when

${\frac{M(t)}{M(\infty)} > 0.4},$

EQ. 9 is dominated by the leading (n=1) exponential, with A₁ and β₁. Asα, and therefore L, become larger, then A_(n) and β_(n) reduce to thedefinitions in EQS. 5 and 6.

Because of strong coupling between β, L, and D in EQ. 9, it can bedesirable to also derive D from the data at short times. In the idealdiffusion case, the diffusion coefficient D was extracted by looking atthe experimental data at short times using EQ. 7. The correspondingequation for EQ. 7 with the boundary condition given by EQ. 8 was notshown by Crank. Accordingly, Laplace transform solutions were derivedthrough terms n=2. For values of

${\frac{M(t)}{M(\infty)} < 0.4},$

EQ. 12 provides a very close approximation of the actual results.

$\begin{matrix}{\frac{M(t)}{M(\infty)} = {{\frac{2}{l}\sqrt{\frac{Dt}{\pi}}} - {\frac{1}{L}\{ {1 - {( {\exp \lbrack {L\sqrt{( {{Dt}\text{/}l} )^{2}}} \rbrack} )( {{erfc}\lbrack {L\sqrt{{Dt}\text{/}l}} \rbrack} )}} \}} + {{higher}\mspace{14mu} {order}\mspace{14mu} {terms}}}} & (12)\end{matrix}$

At short times the higher order terms are small and can be neglected.

Using the above derivation, an example method for calculatingdiffusivity is provided as follows:

-   -   1. Collect experimental data on

$\frac{M(t)}{M(\infty)}$

(the fractional mass loss or mass gain) over time (t) in seconds. For adesorption experiment, this is done by first equilibrating a sample ofknown dry weight to water vapor at a controlled RH and then measuringthe weight of the sample at various times, including its initial valuem(0). Measurements are taken until the weight no longer changes, m(∞).All measurements are taken while blowing dry air over the sample atrates from 10-30 L/min (or about 10-30 L/min) to reduce the effect ofevaporation on the experimental observations.

-   -   2. Calculate the grams of water per gram of dry polymer (W %) at        each time. From this data and other experiments calculate the        value of l(t) at each time, where 2l(t) represents the thickness        of the sample in cm.    -   3. Select initial values (or first estimates) of L and D.    -   4. Define a function G(t) derived from EQ. 12 above according to        EQ. 13:

$\begin{matrix}{{G(t)} = {\frac{M(t)}{M(\infty)} + {\frac{1}{L}\{ {1 - {( {\exp\lbrack {L\sqrt{( {{Dt}\text{/}{l(t)}} )^{2}}} \rbrack} )( {{erfc}\lbrack {L\sqrt{{Dt}\text{/}{l(t)}}} \rbrack} )}} \}}}} & (13)\end{matrix}$

-   -   5. Calculate the values of G(t) using the initial estimates of L        and D.    -   6. Plot G(t) versus

$\frac{1}{l(t)}\sqrt{t}$

and, from the slope of the first four data points, calculate acomparison value for D using EQ. 14:

$\begin{matrix}{{G(t)} = {\frac{2}{l(t)}\sqrt{\frac{Dt}{\pi}}}} & (14)\end{matrix}$

-   -   7. Using the initial value of L in step 3, repeat steps 5 and 6        until D converges. This defines D and L as input parameters for        subsequent steps.    -   8. Using the value of L from step 3, calculate the first six        roots of β_(n) (n=1 . . . 6) according to EQ. 11.    -   9. From experiment, calculate the value of

$\begin{matrix}{\ln \lbrack {1 - \frac{M(t)}{M(\infty)}} \rbrack} & (15)\end{matrix}$

at each time t. From EQ. 9 at longer times, EQ. 15 is equivalent to therelationship given below in EQ. 16:

$\begin{matrix}{{\ln \lbrack {1 - \frac{M(t)}{M(\infty)}} \rbrack} = {{\ln ( A_{1} )} - \lbrack \frac{\beta_{1}^{2}{Dt}}{{l(t)}^{2}} \rbrack}} & (16)\end{matrix}$

-   -   10. Accordingly, the values calculated according to EQ. 15 are        next plotted against

$\frac{t}{{l(t)}^{2}}.$

-   -   11. From the slope of this plot in the range where

$0.35 < \lbrack {1 - \frac{M(1)}{M(\infty)}} \rbrack < 0.65$

and from the value of β₁ calculated in step 8, a new value of D can becalculated using EQ. 16.

-   -   12. Adjust the value of L in step 3 and repeat steps 4 through        11 until the value of D in step 11 and the value of D in step 7        are the same value. This defines unique values for L and D that        satisfy both EQ. 9 and EQ. 12.    -   13. Record values of D, L, A₁, and β_(n), for n=1 . . . 6.        Calculate the full curve using EQ. 9 and calculate and record R²        for the fit. FIG. 20 shows the results of the foregoing        calculation method on an infant-sized foamed polymer tube. The        tube comprises sample MB-27 6% at 52% void fraction, at a flow        rate of 16.7 L/min, RH=100%, D=1.228×10⁻⁶ cm²/s, and L=3.5697.        The R² of the curve fit was 0.9998.

The foregoing description of the invention includes preferred formsthereof. Modifications may be made thereto without departing from thescope of the invention. To those skilled in the art to which theinvention relates, many changes in construction and widely differingembodiments and applications of the invention will suggest themselveswithout departing from the scope of the invention as defined in theappended claims. The disclosures and the descriptions herein are purelyillustrative and are not intended to be in any sense limiting.

What is claimed is:
 1. An expiratory limb for a breathing circuitcomprising: an inlet for receiving humidified gases exhaled by a patientand an outlet for outputting the humidified gases; and a foamed-polymerconduit that is permeable to water vapor and substantially impermeableto liquid water and bulk flow of gas, the foamed-polymer conduitcomprising a water-vapor permeable, solid thermoplastic elastomermaterial with cell voids distributed throughout, the foamed-polymerconduit enabling flow of the humidified gases from the inlet to theoutlet within a space enclosed by the foamed-polymer conduit, and anouter skin comprising a layer of substantially closed-cell foamedmaterial.
 2. The expiratory limb of claim 1, wherein at least some ofthe cell voids in the foamed-polymer conduit are connected to other cellvoids, thereby forming open cell pathways promoting movement of watervapor through the foamed-polymer conduit.
 3. The expiratory limb ofclaim 2, wherein at least 10% of the cell voids in the foamed-polymerconduit are connected to other cell voids.
 4. The expiratory limb ofclaim 1, further comprising a plurality of reinforcing ribscircumferentially arranged around an inner surface of the foamed-polymerconduit and longitudinally aligned along a length of the foamed-polymerconduit between the inlet and the outlet.
 5. The expiratory limb ofclaim 1, further comprising a heating line along a at least a portion ofthe foamed-polymer conduit.
 6. The expiratory limb of claim 1, whereinat least some of the cell voids are flattened along a longitudinal axisextending between the inlet and the outlet.
 7. The expiratory limb ofclaim 6, wherein at least 80% of the cell voids have an aspect ratio oflongitudinal length to transverse height greater than 2:1.
 8. Theexpiratory limb of claim 1, wherein the foamed-polymer conduit has awall thickness between 0.1 mm and 3.0 mm
 9. The expiratory limb of claim1, wherein the foamed-polymer conduit has an inner volume having anaverage void size in the transverse direction less than 30% of a wallthickness of the foamed-polymer conduit.
 10. The expiratory limb ofclaim 1, wherein the permeability P of the foamed-polymer conduit ing-mm/m²/day measured of Procedure A of ASTM E96 (using the desiccantmethod at a temperature of 23° C. and a relative humidity of 90%)satisfies the formula:P>exp{0.019[1n(M)²−0.7 1n(M)+6.5} in which M represents the elasticmodulus of the foamed polymer in MPa and M is between 30 and 1000 MPa.11. The expiratory limb of claim 1, wherein the solid thermoplasticelastomer material has a diffusion coefficient of at least 0.75×10⁻⁷cm²/s.
 12. The expiratory limb of claim 1, wherein the foamed-polymerconduit has a diffusion coefficient greater than 3×10⁻⁷ cm²/s.
 13. Theexpiratory limb of claim 1, wherein the foamed-polymer conduit iscapable of bending around a 25 mm diameter metal cylinder withoutkinking or collapse, as defined in the test for increase in flowresistance with bending of ISO 5367:2000(E).
 14. The expiratory limb ofclaim 1, wherein the foamed-polymer conduit comprises at least 80% ofthe length of the expiratory limb.
 15. The expiratory limb of claim 1,wherein the solid thermoplastic elastomer material comprises acopolyester material.
 16. The expiratory limb of claim 15, wherein thecopolyester material has a polyether soft segment.
 17. The expiratorylimb of claim 1, wherein the permeability P of the foamed-polymerconduit is at least 60 g-mm/m²/day measured of Procedure A of ASTM E96(using the desiccant method at a temperature of 23° C. and a relativehumidity of 90%).
 18. The expiratory limb of claim 1, wherein thefoamed-polymer conduit is configured to be positioned between aventilator and a patient and configured to deliver humidified gas to theventilator from the patient.
 19. An expiratory limb for a breathingcircuit comprising: an inlet for receiving humidified gases exhaled by apatient and an outlet for outputting the humidified gases; and afoamed-polymer conduit that is permeable to water vapor andsubstantially impermeable to liquid water and bulk flow of gas, thefoamed-polymer conduit comprising a water-vapor permeable, solidthermoplastic elastomer material with cell voids distributed throughout,the foamed-polymer conduit enabling flow of the humidified gases fromthe inlet to the outlet within a space enclosed by the foamed-polymerconduit, wherein the foamed-polymer conduit is capable of bending arounda 25 mm diameter metal cylinder without kinking or collapse, as definedin the test for increase in flow resistance with bending of ISO5367:2000(E).
 20. The expiratory limb of claim 19, wherein at least 10%of the cell voids are connected to other cell voids.
 21. The expiratorylimb of claim 19, further comprising a plurality of reinforcing ribs.22. The expiratory limb of claim 21, wherein the plurality ofreinforcing ribs are longitudinally aligned along a length of thefoamed-polymer conduit between the inlet and the outlet.
 23. Theexpiratory limb of claim 19, further comprising a heating line along atleast a portion of the foamed-polymer conduit.
 24. The expiratory limbof claim 19, wherein at least some of the cell voids are flattened alonga longitudinal axis extending between the inlet and the outlet.
 25. Theexpiratory limb of claim 24, wherein at least 80% of the cell voids havean aspect ratio of longitudinal length to transverse height greater than2:1.
 26. The expiratory limb of claim 19, wherein the foamed-polymerconduit has a wall thickness between 0.1 mm and 3.0 mm
 27. Theexpiratory limb of claim 19, wherein the foamed-polymer conduit furthercomprises an outer skin comprising a layer of substantially closed-cellfoamed material.
 28. The expiratory limb of claim 19, wherein thepermeability P of the foamed-polymer conduit is at least 60 g-mm/m²/daymeasured of Procedure A of ASTM E96 (using the desiccant method at atemperature of 23° C. and a relative humidity of 90%).
 29. Theexpiratory limb of claim 19, wherein the foamed-polymer conduit has adiffusion coefficient greater than 3×10⁻⁷ cm²/s.
 30. The expiratory limbof claim 19, wherein the foamed-polymer conduit comprises at least 80%of the length of the expiratory limb.